Cancellation of Contract

I have bought a contract laptop at Mtn Kolonnade on the 16th of Jan 2015. The problem is that there is no signal for it where I stay. I can not access internet, install or download anything. I therefore called MTN to explain this to them. They have then referred me to the coverage dept. I called them on the 20th of January 2015 and they have promised to send someone in my house to assist with the signal problem. I waited to this day, I have not received any call from them. On the 6th of Feb 2015, I wrote a cancellation letter to Mtn requesting them to cancel the contact with immediate effect as it is not working nor assisting me with anything. The administrator requested me to bring the laptop in order for him to send it back to MTN. I indeed took the laptop, modem and everything that came with it in its good condition to the shop. The laptop is now with MTN and not in my possession but still they are refusing to cancel the contract despite the fact that I told them again and again that I do not want the contract anymore. I trust that you will be able to assist me with this problem as I cannot pay for a laptop that does not have a signal. I am so frustrated, please help me cancel.