Hello, due to move from Germany I had to cancel Telekom contract in 2022.I canceled the contract in Telekom shop and returned my rental device there as well. 

I recently noticed that I am still getting charged 5 EUR monthly 2 years after returning the device & canceling the contract.

WhatsApp support is asking for me for the proof but I do not have any, I have moved 2 countries after that point and don't really remember if they gave me any papers confirming the return. 

I provided them the router serial number, how can they don't see in the system that this device is returned?

What can I do at this point?