Hi!
I can now verify that after switch from Tele2 to Telia, my FP3 is back and fully functional:
On Tele2 Network it worked for datatrafic/SMS but not for phonecalls (but using Chattapp (Signal) for voicecalls worked). IMS was NOT registred.
Now on Telia network it is connected on the 4G net, with IMS registerd in *#*# 4636#*#*!
Regarding my Sons FP4 it is funtctioning without issues this far on Tele2 Network.