R198 account accumulate to R11000.00

We had a laptop contract with Autopage for 2gb data per laptop every month at a cost of R198 for 2 laptops. In September 2015 I gave the computer to my son to use, with the understanding that there is 2gb available every moth. Apparently there was no limit on data and I received an account of R11000. How on earth was I not informed that this account can be ran up to such enormous amount?? The provider was MTN. No need to say we destroyed the sim card and will have to pay the money every month until the contract expires. We renewed the contract after two years and had this contract for more that three years, and never was it necessary to pay such an account. I will never ever have any contract with Autopage or MTN again. Autopage was so friendly let me pay it off over 4 months!!