Data Usage for a Top up line

Hi <br> I'm not happy with the service i got from MTNs back office, I got a new contract on the 02 July 2015 for a router line for R169.00 and 2gig day and night express and I was reassured before I walked out the store that the line was placed on a top up line but to my surprise when i got the statement date 14 July 2015 I was billed R1400.00 for the router line and I was charged mostly for data usage, I Called the customer care line on the 22 July 2015 the agent first told me that their statement date ran from 13 June till 13 July 2015,then i told the agent that I only got the number on the 02 of July 2015 and that the line was on top up then she logged a call for me and said i should call back after 7 working days so I called back after 7 working days and i was told the query has not been attended to because they have a back lock and a new service request was logged for me again and was told to call after a week again, I then called after a week and was told the query has been resolved and the billing was true and correct but what I didn't understand was how can i be billed for data usage when I'm on a top up line and another query was logged for me and was told to call after