@Lepotototor Ok start from step one:
boot into android.
enable developer mode system>settings>about phone. Hit Build number a few times to enable developer mode.
Enter developer mode, enable USB/ADB debugging and OEM unlock
4)boot to the bootloader and in fastboot unlock the bootloader
5)boot back into Android and set it up again
6)enable developer options and USB debugging again
copy the downgrade ROM to the root of the device, ie, NOT in a folder
go to system updates and do a local update. It will reboot and you will be on Android 9.04.
enable developer options and USB/ADB debugging again
Unplug and plug the device back in again
Reboot to the bootloader and run TWRP recovery.
In Wipe and advanced wipe, ensure the DATA partition is EXT4
reboot to Android
14)USB/ADB debugging should be enabled
Start the UT installer and follow the instructions.
You have to install Android 9
IF you have installed LineageOS or any other version of Android it will fail to install, it has to be on Android 9
USE the Installer, the fastboot commands are only required to get the device ready for installing UT.
If the device boot loops, usually it is because your DATA partition is not EXT4 or your bootloader is the wrong version (one of the many reasons why you use Android 9).
So you need to get back to Android stock ROM and make sure it is Android 9.
I lack a lot of the understanding of whys and wherefores with the need to set it up in a certain way before installing UT as I am not a software engineer/coder or genius, but if you follow the instructions it should and usually works.
One rule for all of this is keep it simple (KIS), don't complicate things by overusing the command line.