maybe you could try flashing CP (which contains modem partition) from latest beta to see if that works together with final ROM and narrow down if the problem is related to modem firmware or kernel drivers... also older CSC that contains carrier settings could help.Yes, i ave clean flash my divice directly android 11 with factory reset and CSC, not HOME_CSC...
But the problem is alwais there, but not with betas 1 to 6 DBT... Only with beta 7 ,8 and final version... and the last one too...
but please beware, that these mixed versions could not work together so do that at your own risk.