O2 DSL M flex - Contract terminated

Hello O2 Community!

My name is Francesca and in December I've left Germany and moved back to Switzerland.
Before leaving, I have terminated my O2 DSL M (Flex) contract with written notice and returned back the modem. As far as I know, my contract had 1 month of notice period for the termination. However, I have just received the bill for January. What should I do to make sure the contract is no longer active?

In case this helps, my kundennumer is [Kundennummer entfernt]
Thanks everyone who'll be able to help with this.

Cheers,
Francesca

[Kundennummer entfernt - o2_Kurt]