I deactivated O2, but had to pay still afterwards

Hello.
I used O2 DSL M FLEX for a year and then deactivated on 30.November.2018, because I had to return to my home country. The DSL M FLEX was a 12 months contract and I fulfilled it.
The Problem is, I found out that O2 has taken 66,58 Euro from my German bank account a week ago, on 07.02.2019. I got an Online-Rechnung from O2 through my e-mail, and when I checked the 'Rechnung' it said they took 66,58 Euro because they didn't get the "DSL Leihgeräte". (Router)

I sent the Router on 30.November.2018 through DHL. I have the receipt with the Sendungsnummer, and when I checked through DHL Website, it said that the Router was delivered successfully.
Maybe the O2 people didn't check the Router and just took my money.
I hope you can help me with the 'Rücksendeauftrag' information.

Rücksendeauftrag
Auftragsnummer: RET1800456141

O2 Kundennummer: 6042639953

When you can't help me through this information, then please give me an E-Mail address that I can contact about this Problem, so that I can ask and fix this Situation.