Contract cancellation delay

Im just writing this in English because im so frustrated to even contemplate to write in German. 

It has to do with my contract termination.

At mid February 2018 I go to o2 store and i tell them i want to cancel my contract at March 31st. I planned to leave Germany at that very day and did not want my contract to run further that date. The store representative gave me a form and we filled the details together. I get the form and send to o2 that day.

March 31st is my last in Germany and I sent the modem back, just as i was supposed to. 

On April I receive billing. I find this weird and i send them emails to kundendservice but get no reply. Remeber, Im not in Germany and cannot promptly call them. I figure its a mistake and move on

Come June/July, Im always on the move (USA, Canada) and still keep receiving bills. 

What is going on?

I chatted with one sales rep online on July who tells me that they haven't received a letter of cancellation but they received my modem. I tell them that i did not have an undelivered mail from Feb to March 31st. They tell me to send another informal cancellation letter by express mail (see confirmation attached) which i did deliver on July 4th and explicitly stated to confirm me by email when they receive it.

Can you please cancel my contract already? I have not been using your service from April 1st and I keep receiving bills.  It is not fair that I pay for this.

Best Regards