Terminate DSL contract

Hallo! I wanted to use DSL from o2, I made contract online and was told that it is possible to make connection for my address. Then I received router and set technician appointment with o2 contact center. However, when technician came, he fount that it's impossible to make a connection as telekom box is out of order. It's too much effort to fix it and I'm not ready to push this process, so i would like to terminate the contract as it's impossible for o2 to provide me with the DSL service. What should I do for this and how should I return the router back? Thanks.