Incorrect billing

I would like to lay a complaint with regards to how mtn South frica handled my account query and it's affecting my credit score and my account was also suspended.

I had a month to month home router that I was paying between 249-280 I think,last year Black Friday I went to Centurion mall and took a contract for a year for 20mps and was paying 299. The month to month for 10mps was supposed to be cancelled when I took contract for 20mps,this I was told when I made a query around much after realising that there is 2800 that was added to my account but no one was able to tell me where the amount comes from.

I've been calling every month to get an answer but no consultant can explain where the amount comes from,they keep telling me the legal department will call me but they don't,I stopped paying for 3 months thinking that I will get a call back but nothing happened instead they block my home router even if the contract is ending end of October. I just checked my bill now and it shows I owe mtn R5246.39

The balance doesn't make sense at all,let's say since from Jan I didn't pay,the balance owing would be around 2600 not 5k but I've been paying its only 3 month that I haven't pay and it's all because they failed to resolve my account query.