Want my money or data back

I have a contract with Mtn for 40gb per month for 24months and I pay 229 including a router so on the 6th month with them paying according to our agreement they reduced my data to 20gb and have called them to resolve the matter but they haven't now its 12months still they haven't sorted the matter out. They are ****** just want out cause they have failed me while am paying them accordingly