@doniks
ls -l /system

total 88
drwxr-xr-x   2 root root 4096 2020-06-26 06:53 android
drwxr-xr-x   2 root root 4096 2017-01-10 21:46 bin
drwxr-xr-x   2 root root 4096 2016-04-12 20:14 boot
lrwxrwxrwx   1 root root   14 2020-06-26 06:53 cache -> /android/cache
lrwxrwxrwx   1 root root   13 2020-06-26 06:53 data -> /android/data
drwxr-xr-x   3 root root 4096 2020-06-26 05:28 debian
drwxr-xr-x   4 root root 4096 2020-06-26 05:24 dev
drwxr-xr-x 110 root root 4096 1973-06-18 14:58 etc
lrwxrwxrwx   1 root root   16 2020-06-26 06:53 factory -> /android/factory
lrwxrwxrwx   1 root root   17 2020-06-26 06:53 firmware -> /android/firmware
drwxr-xr-x   3 root root 4096 2020-06-26 05:32 home
drwxr-xr-x  19 root root 4096 2020-06-26 05:28 lib
drwx------   2 root root 4096 1970-01-01 00:00 lost+found
drwxr-xr-x   2 root root 4096 2020-06-26 05:23 media
drwxr-xr-x   2 root root 4096 2020-06-26 05:23 mnt
lrwxrwxrwx   1 root root   12 2020-06-26 06:53 odm -> /android/odm
drwxr-xr-x   3 root root 4096 2020-06-26 05:31 opt
lrwxrwxrwx   1 root root   16 2020-06-26 06:53 persist -> /android/persist
drwxr-xr-x   2 root root 4096 2016-04-12 20:14 proc
drwx------   2 root root 4096 2020-06-26 05:32 root
drwxr-xr-x   8 root root 4096 2020-06-26 05:31 run
drwxr-xr-x   2 root root 4096 2020-06-26 05:32 sbin
drwxr-xr-x   2 root root 4096 2020-06-26 05:23 srv
drwxr-xr-x   2 root root 4096 2016-02-05 09:48 sys
lrwxrwxrwx   1 root root   15 2020-06-26 06:53 system -> /android/system
drwxrwxrwt   2 root root 4096 2020-06-26 05:32 tmp
drwxr-xr-x   2 root root 4096 2020-06-26 06:53 userdata
drwxr-xr-x  10 root root 4096 2020-06-26 05:23 usr
drwxr-xr-x  11 109  113  4096 2020-06-25 12:57 var
lrwxrwxrwx   1 root root   22 2020-06-26 06:53 vendor -> /android/system/vendor

Should I reinstall UBPorts?

@Capsia Okay, based off of that, it seems like I am not in the terminal, unfortunately. Is there anything else I could try?

@joe The permissions were -rwxrwxrwx 1 root root, but that should work as well (albeit it could be more secure). The profile has the same name as the SSID.

After testing, I don't think I was in the terminal. Should I try reinstalling UBPorts from scratch, and is there an initial setup page that I haven't bypassed that I may be hitting? I'm trying to figure out why I didn't access the terminal. I believe I followed the instructions exactly.

@joe I tried copying the file from my Manjaro machine, replacing the permissions and adding the password, but it didn't seem to work, as I couldn't detect my phone on nmap from another computer on the LAN. I even tried to get into the terminal to use nmcli to connect (https://docs.ubuntu.com/core/en/stacks/network/network-manager/docs/configure-wifi-connections) but it didn't seem to work, either. How can i get my internet connected, and is there any way to verify I'm actually in the terminal? Sorry for how complicated this is getting, and thanks for the help.

@joe Okay, I think nearly everything is setup for SSH. The one thing I can confirm isn't working yet is wifi. My network runs on wpa-psk, and I need some way to bring the credentials to the phone. Is this possible through adb shell?

EDIT: Also, do I need to TAB when opening the terminal before entering my password? Thanks.

@doniks
Ah, I didn't add sbin, /usr/bin, and /usr/sbin to PATH. passwd was in /usr/bin. I changed the password, so I think I'm able to get in. But, it seems like adb isn't even supported on the OP3T, so it seems like a moot point. @joe since you were able to help me navigate to developer mode, do you think you can tell me how to open the terminal with a keyboard, so I could try to setup SSH?

@doniks
ls -l /system

total 88
drwxr-xr-x   2 root root 4096 2020-06-26 06:53 android
drwxr-xr-x   2 root root 4096 2017-01-10 21:46 bin
drwxr-xr-x   2 root root 4096 2016-04-12 20:14 boot
lrwxrwxrwx   1 root root   14 2020-06-26 06:53 cache -> /android/cache
lrwxrwxrwx   1 root root   13 2020-06-26 06:53 data -> /android/data
drwxr-xr-x   3 root root 4096 2020-06-26 05:28 debian
drwxr-xr-x   4 root root 4096 2020-06-26 05:24 dev
drwxr-xr-x 110 root root 4096 1973-06-18 14:58 etc
lrwxrwxrwx   1 root root   16 2020-06-26 06:53 factory -> /android/factory
lrwxrwxrwx   1 root root   17 2020-06-26 06:53 firmware -> /android/firmware
drwxr-xr-x   3 root root 4096 2020-06-26 05:32 home
drwxr-xr-x  19 root root 4096 2020-06-26 05:28 lib
drwx------   2 root root 4096 1970-01-01 00:00 lost+found
drwxr-xr-x   2 root root 4096 2020-06-26 05:23 media
drwxr-xr-x   2 root root 4096 2020-06-26 05:23 mnt
lrwxrwxrwx   1 root root   12 2020-06-26 06:53 odm -> /android/odm
drwxr-xr-x   3 root root 4096 2020-06-26 05:31 opt
lrwxrwxrwx   1 root root   16 2020-06-26 06:53 persist -> /android/persist
drwxr-xr-x   2 root root 4096 2016-04-12 20:14 proc
drwx------   2 root root 4096 2020-06-26 05:32 root
drwxr-xr-x   8 root root 4096 2020-06-26 05:31 run
drwxr-xr-x   2 root root 4096 2020-06-26 05:32 sbin
drwxr-xr-x   2 root root 4096 2020-06-26 05:23 srv
drwxr-xr-x   2 root root 4096 2016-02-05 09:48 sys
lrwxrwxrwx   1 root root   15 2020-06-26 06:53 system -> /android/system
drwxrwxrwt   2 root root 4096 2020-06-26 05:32 tmp
drwxr-xr-x   2 root root 4096 2020-06-26 06:53 userdata
drwxr-xr-x  10 root root 4096 2020-06-26 05:23 usr
drwxr-xr-x  11 109  113  4096 2020-06-25 12:57 var
lrwxrwxrwx   1 root root   22 2020-06-26 06:53 vendor -> /android/system/vendor

Should I reinstall UBPorts?

@doniks Okay, I'll try it out. The PATH thing was a typo on my part; I did PATH=$PATH:/bin, but it seems like I didn't have the right idea from the start, so I'll try your instructions. Thanks

Edit:
ls -l /

total 3372
dr-xr-xr-x   4 root root         0 1970-01-04 02:12 acct
lrwxrwxrwx   1 root root        11 1970-01-01 00:00 bin -> /system/bin
lrwxrwxrwx   1 root root        50 1970-01-01 00:00 bugreports -> /data/user_de/0/com.android.shell/files/bugreports
drwxr-xr-x   3 root root        60 2017-01-12 13:56 cache
lrwxrwxrwx   1 root root        13 1970-01-01 00:00 charger -> /sbin/charger
dr-xr-xr-x   2 root root        40 1970-01-01 00:00 config
lrwxrwxrwx   1 root root        17 1970-01-01 00:00 d -> /sys/kernel/debug
drwxr-xr-x   8 root root      4096 2017-01-09 11:51 data
lrwxrwxrwx   1 root root        12 1970-01-01 00:00 default.prop -> prop.default
drwxr-xr-x  10 root root      3760 1970-01-04 02:12 dev
drwxr-xr-x   2 root root       140 2017-01-12 13:56 etc
-rw-r--r--   1 root root     33643 1970-01-01 00:00 file_contexts
-rw-r--r--   1 root root    674825 1970-01-01 00:00 file_contexts.bin
-rwxr-x---   1 root root   2212400 1970-01-01 00:00 init
-rwxr-x---   1 root root      3114 1970-01-01 00:00 init.rc
-rwxr-x---   1 root root       103 1970-01-01 00:00 init.recovery.hlthchrg.rc
-rwxr-x---   1 root root        52 1970-01-01 00:00 init.recovery.ldconfig.rc
-rwxr-x---   1 root root        35 1970-01-01 00:00 init.recovery.mksh.rc
-rwxr-x---   1 root root      5727 1970-01-01 00:00 init.recovery.qcom.rc
-rwxr-x---   1 root root       111 1970-01-01 00:00 init.recovery.service.rc
-rwxr-x---   1 root root      1323 1970-01-01 00:00 init.recovery.usb.rc
drwxr-xr-x   3 root root        60 1970-01-01 00:00 license
drwxr-xr-x   3 root system      60 1970-01-04 02:12 mnt
drwxr-xr-x   2 root root       200 1970-01-01 00:00 odm
drwxr-xr-x   2 root root        40 1970-01-01 00:00 oem
drwxrwxrwx   2 root root        40 1970-01-04 02:12 persist
-rw-r--r--   1 root root     23786 1970-01-01 00:00 plat_file_contexts
-rw-r--r--   1 root root      7212 1970-01-01 00:00 plat_hwservice_contexts
-rw-r--r--   1 root root      6687 1970-01-01 00:00 plat_property_contexts
-rw-r--r--   1 root root      1315 1970-01-01 00:00 plat_seapp_contexts
-rw-r--r--   1 root root     14057 1970-01-01 00:00 plat_service_contexts
dr-xr-xr-x 384 root root         0 1970-01-01 00:00 proc
lrwxrwxrwx   1 root root        15 1970-01-01 00:00 product -> /system/product
-rw-r--r--   1 root root      3042 1970-01-01 00:00 prop.default
drwxr-xr-x   3 root root        80 1970-01-01 00:00 res
drwx------   2 root root        40 2019-04-15 10:55 root
drwxr-x---   2 root root      6700 2017-01-12 13:59 sbin
drwxrwx---   3 root root      4096 2017-01-09 11:51 sdcard
-rw-r--r--   1 root root    387957 1970-01-01 00:00 sepolicy
drwxr-xr-x   2 root root        40 1970-01-04 02:12 sideload
drwxr-x--x   2 root root        40 1970-01-01 00:00 storage
dr-xr-xr-x  12 root root         0 1970-01-04 02:12 sys
drwxr-xr-x  24 109  113       4096 2017-01-10 21:47 system
drwxrwxr-x   2 root shell       80 2017-01-12 14:01 tmp
drwxr-xr-x   5 root root       180 1970-01-01 00:00 twres
-rw-r--r--   1 root root      5272 1970-01-01 00:00 ueventd.rc
drwxrwxrwx   2 root root        40 2017-01-12 13:56 usb_otg
drwxr-xr-x   3 root root        60 1970-01-01 00:00 vendor
-rw-r--r--   1 root root      6178 1970-01-01 00:00 vendor_file_contexts
-rw-r--r--   1 root root         0 1970-01-01 00:00 vendor_hwservice_contexts
-rw-r--r--   1 root root       218 1970-01-01 00:00 vendor_property_contexts
-rw-r--r--   1 root root         0 1970-01-01 00:00 vendor_seapp_contexts
-rw-r--r--   1 root root         0 1970-01-01 00:00 vendor_service_contexts
-rw-r--r--   1 root root        65 1970-01-01 00:00 vndservice_contexts

cat /etc/fstab

/dev/block/sda3 /cache ext4 rw 0 0
/dev/block/sda15 /data ext4 rw 0 0
/dev/block/sda2 /persist ext4 rw 0 0
/dev/block/sde20 /system ext4 rw 0 0
 /usb_otg vfat rw 0 0

mount

'rootfs on / type rootfs (rw,seclabel,size=2834740k,nr_inodes=708685)
tmpfs on /dev type tmpfs (rw,seclabel,nosuid,relatime,size=2918948k,nr_inodes=729737,mode=755)
devpts on /dev/pts type devpts (rw,seclabel,relatime,mode=600)
proc on /proc type proc (rw,relatime,gid=3009,hidepid=2)
sysfs on /sys type sysfs (rw,seclabel,relatime)
selinuxfs on /sys/fs/selinux type selinuxfs (rw,relatime)
tmpfs on /mnt type tmpfs (rw,seclabel,nosuid,nodev,noexec,relatime,size=2918948k,nr_inodes=729737,mode=755,gid=1000)
none on /acct type cgroup (rw,relatime,cpuacct)
tmpfs on /tmp type tmpfs (rw,seclabel,relatime,size=2918948k,nr_inodes=729737)
pstore on /sys/fs/pstore type pstore (rw,seclabel,relatime)
adb on /dev/usb-ffs/adb type functionfs (rw,relatime)
/dev/block/sda15 on /data type ext4 (rw,seclabel,relatime,data=ordered)
/dev/block/sda15 on /sdcard type ext4 (rw,seclabel,relatime,data=ordered)
/dev/block/sde20 on /system type ext4 (rw,seclabel,relatime,data=ordered)

mount /data

mount: '/dev/block/sda15'->'/data': Device or resource busy

ls -l /data

1|OnePlus3:/ # ls -l /data
total 32792
-rw-------  1 root   root   33554432 1973-06-18 14:57 SWAP.img
drwxrwx--x 44 system system     4096 1973-06-18 14:58 android-data
drwxr-xr-x  3 root   root       4096 1973-06-18 14:57 cache
drwx------  2 root   root       4096 1970-01-01 00:02 lost+found
drwxrwx---  3 root   root       4096 2017-01-09 11:51 media
drwxr-xr-x  5 root   root       4096 1973-06-18 14:58 system-data
drwxr-xr-x  3 root   root       4096 2020-06-26 05:32 user-data

The first step of the guide you linked involves the following the command: mount /data/rootfs.img /a, which outputs that rootfs.img does not exist.

@doniks I booted into TWRP. Opened up the adb shell and did the following:

twrp mount system
chroot system
PATH=$PATH:PATH
passwd phablet

And I got /bin/sh: 3: passwd: not found

Since it doesn't seem like I'll set up adb or ssh, any suggestions to run headlessly on my phone? Thanks.

I've tried all of the suggestions so far. I don't know if a password is on my default, and 0000 didn't work (also is there an initial setup I have to complete before being able to perform this action). I chrooted in TWRP, but passwd was not in the /system/bin directory. So, I'm making progress, but am not there yet.

@joe Thank you so much. The only thing is that I don't have a password, which is supposedly required for enabling the mode. Also, how would I accept my computer's keys for adb? Sorry for all of the complications.