I managed to fix the radio issue and the battery drain (which corroborates my suspicion that the battery drain was caused by the radio module).
I first did a fresh install of the same e/OS version (2.1-t) which did not help.
But flashing a fresh install of fairphone OS 13 (FP4-TP2D-factory) which absolutely has to have the same Security Patch Level then whatever you have installed (beware of bricking), both radios worked well. Interestingly my eSIM survived the whole endeavor.
After confirming it fixed with fairphone OS, I reflashed e/OS 2.1-t and the issue is fixed.
I don’t exactly know what the issue was, so I cannot recommend this procedure generally.
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