DSL router not working

Hello,  My DSL router is not working since one week. I have tried resetting it many times. It does not accept the PIN as well.  I contacted many times technician through call and got no response. I wrote a letter to o2, got no response. I am continuously asking to o2 Deutschland guys on facebook, no response after saying that they generated ticket.    I do not know how and whom to reach for my request. This is the height of tolerance.  I want to cancel my contract now. Please guide me how to cancel the existing contract. I bought it on 23 March 2015. I will be thankful if somebody can guide me through the cancellation procedure only.  

Number: 0511XXXXXXXX

Xxxxxx Xxxxxxx

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