O2 account and bill

Dear O2 team,

I had a contract with you from july 2017 to march 2018. I personally cancelled this contract in march 2018 and sent the router back to the requested address. However, I just realized that after I cancelled the contract and payed the last bill on april 3rd 2018, I received a new bill for 4.35 Eur on april 30, 2018, when I had already left Germany and cancelled my bank account. So, I want to know the following:

1. was my contract really cancelled?
2. if so, why are you charging me that?
3. why is this amount increasing? I just got an email saying that I "owe" now 26.85 Eur for something I didn't use.

I hope you can please help me solve this problem because I am not in Germany anymore nor pretend to go back soon.

Regards,

Federico Padilla Gonzalez
Kundennummer XXXXXXXXXX

Edit o2_Giulia: This is a public forum. Please don't post personal data like phone or customer numbers.