Bad service

I entered into a contract with Mtn around March for a package of a router and a Sim card which costs me close to R450 every month but the Sim card does not work all the time, the signal is always off and every time I call them they keep saying the can't assist me. I asked them to cancel my contract but they said cancelation fee is almost R2000 and mind you I am an Intern and that's almost half my salary now I have to keep on the contract while they deduct money but I don't get the services I pay for.