@matteo I could never really get the PDK to work reliably on my Ubuntu laptop either, to be honest.
But I guess a good place to ask for help would be the Developer group chat on Telegram

Could this have something to do with the problem we're seeing? https://github.com/ubports/ubuntu-touch/issues/1837 If so, it's not obvious to me there is a solution. @UniSuperBox posted "Funny enough, you can get another shell into the device in this state. Running sudo reboot there causes the Pixel 3a to boot into the selected mode, but the Volla Phone does not." but I don't know how to use this information to complete the install. BTW: I tried using UBports 0.8.8 ... but I ran into the same problem.

@matteo The first is a known halium 9 problem on multiple devices.

The second is because no one has come up with a splash image yet to replace it, as the device is relatively new to the installer.

@matteo
Any news here ?
Check there if you can : https://forums.ubports.com/topic/9213/mobile-network-data-counter

Small update.
After the PRs by @UniSuperBox and @jonius I was finally able to compile the keyboard package onto my phone. In order to accomplish the task I had to slightly modify the command I used before, i.e. crossbuilder with crossbuilder --password=myPro5password because a bunch of commands sent to the phone requires sudo. In this way I was able to get to the end of the crossbuilding process, I believe without errors. At first I changed just a small thing in the code to see the effect on the phone, but then I tried without changing a thing and after a reboot in both cases I got the keyboard not showing up anymore; so I couldn't access the phone because I couldn't insert the unblocking passcode. A fresh UT install did the trick. Any idea why the crossbuilding process seems not working as expected? I don't have the output log this time but, if needed, I can try again to compile and collect a new one.