Okay I advanced one step.

1. For the computer to see the device when in fastboot, it needs a different driver. Go and download it here: https://developer.android.com/studio/run/win-usb. Unzip the driver file somewhere you can remember.
2. Turn off phone.
3. Start it in fastboot (hold Volume Down + Power button).
4. Now in Windows open Start Menu and type devmgmt.msc, and locate Other devices > Android device. Right-click this and select Update driver.
5. Point Windows to the driver folder you unzipped earlier.
6. Confirm. The driver gets installed.
7. Now in the Windows Device Manager (devmgmt.msc), you should see Android Device > Android Composite ADB Interface. This is what the phone looks like to your computer when the phone is in fastboot state.
8. The ubports installer will now detect the phone when needed.

Ubuntu Touch was installing while I was writing this and it is now booting in there for the first time! Language selection screen is up!

Good luck

Okay I advanced one step.

1. For the computer to see the device when in fastboot, it needs a different driver. Go and download it here: https://developer.android.com/studio/run/win-usb. Unzip the driver file somewhere you can remember.
2. Turn off phone.
3. Start it in fastboot (hold Volume Down + Power button).
4. Now in Windows open Start Menu and type devmgmt.msc, and locate Other devices > Android device. Right-click this and select Update driver.
5. Point Windows to the driver folder you unzipped earlier.
6. Confirm. The driver gets installed.
7. Now in the Windows Device Manager (devmgmt.msc), you should see Android Device > Android Composite ADB Interface. This is what the phone looks like to your computer when the phone is in fastboot state.
8. The ubports installer will now detect the phone when needed.

Ubuntu Touch was installing while I was writing this and it is now booting in there for the first time! Language selection screen is up!

Good luck

Hi, exact same situation here. Stuck at @NewbyNoBrain's list's #9.

On my phone (hammerhead D821(H)) the fastboot page's last line says: "LOCK STATE - unlocked".

Basically my computer seems to detect the phone whenever Android is booted and running (command adb.exe devices returns the phone's ID), but nothing happens when it's in fastboot.

Thanks for your help.