The Bundesnetzagentur runs a free consumer arbitration procedure for telecom disputes. Bring your evidence.
It costs nothing, it is designed for exactly this situation, and providers take it seriously.
The Verbraucherzentrale also offers paid legal advice if the matter is more complicated.
Gather your cancellation, your proof of sending, and any correspondence.
File with the Bundesnetzagentur.
Do this while you still have a German address if possible.
The Bundesnetzagentur runs a free consumer arbitration procedure for telecom disputes. Bring your evidence.