Money taken from bank after termination

Dear all,

Sorry for posting here, but I can't post in the English group.

I canceled my O2 DSL contract end of June because I left Germany.

However, I just saw that in September they have charged me 44.54 euro.

I have not received any bill on my new address that I gave O2 and my online account has already been deactivated. Therefore, I cant see my online bills.

I paid all my bills in time and I also sent back the router in time (on the day that my contract ended).

Does anybody know how should I proceed?

Thank you in advance,

Best,

amaia