Contract Cancel

I am very angry about the work efficiency and service quality of O2. I made the contract on 2nd Oct, and an appointment for Internet connection on 23th Oct. But I only got an E-mail from O2 , said the connection is not successfull. I have tried many times to call the hotline but can’t get in. And the workers in O2 shop only said, Telekom technitian found some error of my house kabel but didn’t tell me when the problem can be solved. I have to wait and now already 1 month , I still have no access to internet. So I told the O2 worker that I want to cancel the contract, but got the answer, only by writing a letter to O2 to cancel it. I think it is unbelievable in Germany, such an inefficient way to solve the problem. Can someone tell me, how can I cancel the contract? Is there extra Fee for cancellation? I think I only get the router but don’t use any Internet , so the cancellation should be free.