DSL contract cancellation

Hello O2 team,

I have been trying to cancel my O2 DSL contract since February and am not able to do so.

On March 31st I moved out of Germany. Before moving out I sent a cancellation letter to O2 (mid February) and also returned the modem (March 31st).

In April I still received a bill from you so I contacted customer service and they said you have not received my cancellation letter from February. I have sent two more letter of cancellation for which I have proof from the post office that you have received them. However, I keep getting bills and the customer service I contact via chat still tells me you have not received my letters. I have sent friends to your store in Munich to solve the issue, since I now live abroad, but the store cannot cancel my contract either.

Please let me know how I can cancel the contract in a way that assures me that you do receive my cancellation. At this point I cannot send letters anymore since you keep saying you don't receive them. What other options do I have besides sending letters?

I am not sending my kundennummer here since it is a public post but let me know if you need my personal information.

Thank you very much for your assistance!
I look forward to your feedback.