Termination of mobile contract M-Flex

I contracted this plan because I thought that I could cancel it easily anytime after 6 months. I was wrong!!!!

When I went to the shop, they told me to send the documents to an specific address... this is getting complicated!

OK, I sent the documents (einschreiben) on September 30th, the package was received on October 4th, but I am still paying!

Handynummer: 0176xxxxx808

Now I am in Mexico, sending a letter costs 60 euros and calling is also expensive!

What can I do?

EDIT LMSMDK: Rufnummer gekürzt. Datenschutz.