MTN charges extra fees on top op internet contract

I took out a top up contract at MTN for a wi-fi modem. My understanding of the top up contract was that once the data received for the specific month is depleted, I would have to by more data out of my pocket for further usage until the next month's data is received. This was my understanding as I set a limit on the usage of my monthly data. For a few month's thereafter, this was the case, however, as from August 2015, this limit seemed to have disappeared as MTN was now charging me additional costs that was way, yes way more than my usuall subscription. I reported this matter and after a week, there was no response from MTN, after they telephonically promised me that the matter will be receiving attention. After nothing I decided to contact them again and again lodged a complain with them. I hope I will receive at least a response this time round.