R599 MTN data contract billed at double the agreed upon price.

Took a MTN "made for home" contract in September for their 210GB deal with a LTE router. Contract price - R599. It was explained to me that the first payment will be higher because I need to pay for data received before the first debit order. No problem. First debit order - end of September: R1500. Now I receive an invoice that October debit order is R1299!! This is *****. Of the 210GB data I only managed to use 34GB for the month because MTN has no internet connectivity, I would estimate, 50% of the time. The router connects just fine and indicates "connected" but when you ping Google or try to surf the net, there is no internet connection from MTN to the rest of the www. I've now started collecting data using a small app that pings Google every 10 seconds and keeps a data log of connectivity. I fully intend to sue MTN for breach of contract for debiting more money than agreed upon and failing to provide a reasonable "always on" internet connection. I've learned to expect to be disappointed with South African telecom companies, but THIS is just plain theft.