@RJDan I don't have a Fairphone 5, but on the devices page for the Fairphone 5
https://devices.ubuntu-touch.io/device/fp5/ , it is said that on "see how to install UT" :
"Your device must be running the Fairphone OS version of Android 13 before installing another operating system. With a previously installed custom ROM, it won't work! "
But yes, wait a confirmation of the procedure of somebody who flashed UT on Fairphone 5...