REFUND required from MTN Retention department ASAP!

I took out a fixed WiFi @ home package in the beginning of August 2018. The WiFi stopped working at the end of August. After countless MTN visits and customer service phone calls it could not be fixed. The router with all components in the original package was returned to MTN Walmer Park, Port Elizabeth branch. The debit order of R1000 has still been going off monthly. As there were no services rendered I should be reimbursed for 6 months of payment. Since it is impossible to get hold of retention department, how do I obtain my refund? Mtn should not provide services they CANNOT deliver on. I want my refund!