@Lakotaubp Thanks for the quick reply, I found help on the telegram group.

To enter fastboot I had to

sudo reboot -f bootloader

in the UT terminal

I installed UT using the installer, selected the 20.04 rc channel, but now I want to switch to a different version. I have enabled developer mode.
Using

adb reboot bootloader

results in the screen turning off, but nothing else happens. The terminal is stuck on that command until I ctrl+c.
Trying to enter fastboot using volume up+power is stuck in a loop of splash screen to "bootloader unlocked" message then back to splash screen for as long as I hold the buttons. When I release them it just boots normally. Holding volume up+volume down+power results in splash screen flashing for a moment and then disappearing and then flashing again for as long as I hold the buttons. Again releasing them results in UT booting normally.
When I try to use the installer and select FP4 manually it also gets stuck on rebooting to bootloader.