Okay I advanced one step.
- 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.
- Turn off phone.
- Start it in fastboot (hold Volume Down + Power button).
- Now in Windows open Start Menu and type devmgmt.msc, and locate Other devices > Android device. Right-click this and select Update driver.
- Point Windows to the driver folder you unzipped earlier.
- Confirm. The driver gets installed.
- 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.
- 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!