Thanks for the offer to fix issues. I’ve encountered one:
I’m a blau.de customer which uses the E-Plus (and now also O2) network in Germany. The roaming issues was fixed by Fairphone as discussed in this thread.
Unfortunately I can’t give you mmc/mmn data since I don’t know what that is (and googling didn’t help). My only guess is that this is a typo and you mean MCC/MNC. These seem to be the following for my provider blau.de (second source):
MCC: 262
MNC: 03
I don’t know whether this is the correct information to make usage of both O2 and E-Plus network work, though.
I’d be happy to hunt for more information if necessary.