Charging me R2100 this month on a R400 contract

Have a tablet/data contract @ R399/ Month which will be debited for R2100. They claimed I went over my data limit which is impossible as i had not even used any data. When queried they claimed i had gone over my data and will be charged the R2100. I queried again to have them tell me I had not used my data and should not be charged the R2100 but my normal rate only they will still be debiting the R2100 on the first of next month but will pay back the difference into my account which may take up to 3 weeks. It has not even been debited yet but must still be debited. So why not change the debit? I first queried this in the beginning of the month and only now got feed back. The service is shocking and not even so much as an apology for this. Although I did say I would not be continuing this contract once it expires and was told by the manager that if I go to another network I will just get other problems, hopefully though they won't be charging me 5x my rate. All in all I have several Mtn contracts and feel they could at least try to assist me better and keep me up to date without me having to beg for information.