Contract cancellation

I got a router from MTN last year November and it wasn’t working at all. I went to all mtn stores at alberton and they told me they can’t help because I ordered online. I called Mondo Customer Care since they’ve Been the ones contacting me and I told them I want to cancel the contract firstly because they sold me something we did not agree on and secondly it’s not working. They agreed to cancel and gave me the *** and they sent a courier guy to fetch the router December. Till today they still billing me and sent my account to css and also it’s messing up my credit score