Shocking account bill

At end of July while on vacation in Balito I decided to upgrade both my cell and data bundle as I had time on my hands. When I got the bill for August I had a shock as I was billed R3602 for the Data and the bill normally comes to R155. Back home I went to MTN branch at Carnaval mall to investigate the problem. The queries was put forward to head office from the branch and still to date I have not heard anything. Now come September bill....now almost had an heart attack.....Bill of R5793....cant believe it as my data usage was more or less the same monthly. Went back to MTN at Carnaval Mall and they found out that I had the open contract. I found it to be so unfair as my contract with mtn for the last 12 years was always the same and was capped. And MTN is taking so long with my queries and going into the second month I was punish with a massive bill. Now I don't know what to do as it is way out of my reach and I am so scared to used the data...and don't want a massive heart attack next month. I am a deaf person with limited ability to have a conversation on the phone but find the tech of whattsapp,sms and email very handy.