Delayed to return the router

I have been dragged by MTN because I once ordered a router via online. I have paid them for first 2 months then I stopped because I am not getting any service from the router. I also gave them a call once so that I can return the router them. They then, told me that i can return the router to any store, when I went to a certain mall I was told to go to certain places meaning their other stores. I am feeling frustrated because it's been more than a month, and I am also told that I don't pay before the 25th of this month they will charge me about to 2 thousand rand for the service I am not getting. I feel like I am paying for free and they expect to pay the remaining amount to terminate the contract.