Unrealistic internet billing and poor service

My internet billing statement was very high in July 2015 (R1366.50), on 3 August I went to Mtn branch in The Glen requesting the GB usage for the previous month. The response was that Mtn cant give me the exact usage but my internet usage was high and that I made a lot of data calls (They could not explain to me what they meant by data calls). When asking what I should do to manage my usage I was then advised to migrate to a Top Up contract and my maximum billing would be R 129.00 per month. The migration documents was done the same day and the responsible person said a request was send to do the migration. My understanding was that the issue was resolved till I got my next statement and for the month of August I used R 4174.76 of data (This is equal to 4 time 20GB day time and 20GB night express data contacts). This is after my router was switched off most of the time, my cell phones was not connected to the router, I changed my router password so that some stranger cant use my internet and I was using a CellC prepaid router for most of the month of August). When I went to The Glen branch on 25th August I was told that they have no migration documents.HOW MUCH DATA IS R4174.76