Laptop not working and MTN is ignoring me!

I got into a contract with Mtn for a laptop in December. This was done through a consultation who the couriered the laptop. I received my laptop mid December and was not able to use it because I was busy. In Jan, when I started using it, the laptop was just too slow, had a Lenovo screen saver, was taking to long to open and too long to open documents and the login in was not onto the computer working worked occasionally. I called MTN and was told to take it to any mtn dealership. When I did I was sent to Morningside, and then morning side refused to help me an gave me a number to call. I called and the arrangements to come and get the laptop for repairs. Laptop went in for about 3weeks and laptop is back and everything else is fixed except for the laptop is now super slow. I have to wait over 5mins for a document to open or a page to open. I sent the consultant an email to get me details of the supervisors so I can terminate this contract and no response. I don’t see why I’m paying R390 every month for a laptop that not working. I constantly have to borrow my son’s laptop so I can work. I want to terminate the contract please, I’ll got find a better laptop somewhere else!