I had the same issue some weeks ago with my FP3+ on stable Focal with Free operator in France.
I suddenly lose data connection without any event or explanation. Before that it was perfect. After, data was available for about 30 sec after reboot and then get lost. I tried so much things I can t remember. Maybe some of you may remeber that period in the French Telegram group...
I reset the apn, did reboots on diverses manners, changed sim slot, changed sim card, wipe phone to reinstall various images (xenial, focal, stable, dev...), switch back to android (data worked!) and back to UT (no data again!), tried a ton of stuff in the terminal thanks to support from the French community, etc. Nothing ever worked for me!
After about 1 month I remembered some advice from the Fairphone tutorial. When you reinstall Android, you have to do it without a sim card in your phone. I never did that to install Android or UT and never had a pb.
But that time I did reinstall Android without a sim card on the phone, then put the sim and tested data connection, and then reinstall UT without the sim and... That time it worked! So that might be the solution for you also.