Not sure if it helps but I just installed ubuntu touch on this device from Android. Because I ran into some issues on 18.04 with apparmor I installed it from a live usb of 16.04.
First I used https://docs.ubports.com/en/latest/userguide/install.html#install-on-legacy-android-devices to get into the old ubuntu touch.
Then on 16.04 Live, I ran into some dependency problems which could be solved by enabling the universe/multiverse and restricted channels via software and updates. Manually install adb and fastboot using:
sudo apt install android-tools-adb android-tools-fastboot
Or let aptitude solve all dependencies for you (after installing aptitude via sudo apt-get install aptitude)
Now my tablet was not recognized despite the developers mode on, I had to select manually twice before I could enter install mode. First attempt went wrong but I believe this was a password issue, second attempt went like expected.
Since this install was from live it limits the amount of variables which could spool the fun, so maybe it helps you also, or points more precisely to what goes wrong.