Contract manipulation

My partner and I got a sim only data contract from MTN which is R200 p/m and for the first two months everything was going well then the third month we received a bill of R509 rather than the R200 which our contract installment was and she went to MTN to check and found out that we had spent more data then the plan was for so we went over our limit, we accepted this and paid the R509 and then went on the MTN app and set our limit to R1 over our R200 payment so after R201 worth of usage we should have no access to internet, then the following month we received another bill of R509 and when my partner went to the MTN store no employee wanted to assist her and no manager was present to help, we have not paid this amount along with two other additional months and we have not used the card since because no one is owning up to the issue which they caused and when we want to terminate the contract they say we must pay when the fault is on their part. We have since sent the matter to them via email and will send it to ICASA soon because we used this sim for our internet cafe and we have been struggling without proper internet access plan and yet no one wants to fix this issue.