I still don't know how to make the sdcard appear directly on my desktop computer when I connect my phone, but I have a workaround : symlink the sdcard mount point in the /home/phablet directory ( = "internal storage" on the desktop computer).

This is easily done using the terminal (or adb shell). Find the exact mount point of the sdcard using the command mount; it is something like /media/phablet/XXXX-YYYY. Then enter ln -s /media/phablet/XXXX-YYYY ~/sdcard

Since it was working in android, there must be a way to have it working in UBports. I had more or less the same issue in the Xperia X port and could make it work (see here). Turns out that the proper device to use was usb0 and not rnids0 (but editing some files was still required to have usb0 being available). If you are comfortable with linux command line and modifying bash scripts, you should try things along what I did.

@gpatel-fr thanks for digging into this. So, right, it's supposed to use ncm and not rndis, but usb-moded should handle tethering with that .
The file you tried to cat is restricted to root access, but even with sudo, it's not accessible, and that probably explains why the configuration script fails, falling back to charging only.
Maybe the fp5 is missing the corresponding kernel module now, while it had it back in August?
I created a ticket.