Got it back to 20.04. Pretty simple if you do a lot of ROM installations:
Download TWRP for for 5T (Dumpling) here. Attach the device to your PC in fastboot mode (bootloader) and install TWRP from the command line in the PC -
./fastboot flash recovery <location of your TWRP file>
Boot into recovery, ensure the data partition is EXT4 and wipe cache and system. Reboot the phone to recovery.
Download the stock ROM (OnePlus5TOxygen_43_OTA_004_all_1711152324_9a7ca8ad1c7d.zip) from here. Copy it to the device NOT in a folder so it is in the root of the device.
In TWRP choose install and install the ROM zip file - leave it alone and let it do its thing. Once installed reboot the phone and again leave it alone, let it do its thing. Run through Android set up and then enable developer options and USB debugging.
Run your UT installer and follow the instructions.