Deleting my debit order details and now refusing to cancel a month to month contract

MTN has been my fibre provider for a few months, and while the product itself has been sufficient, their back office management has been an unmitigated nightmare.

When I called, and emailed in July to give notice to cancel my contract at the end of August, I was told my account was in arrears. This was odd since I gave them details for debit orders. The first debit order went off smoothly and I assumed all was going off as normal from there.

Turns out they "cancelled" my debit order details due to the credit card details I provided having expired. Now I know it isn't the credit card itself because that only expires in 2018.

I have been trying, unsuccessfully, for a month now to get a detailed recon and confirmation that I can pay the last few months of debit orders via credit card. MTN is not responding, and the latest is that they will not cancel my month to month contract, which I gave at least 30 days notice of, due to this outstanding amount.

Is it not a violation of the NCA to refuse to cancel a contract under these circumstances? Is this a ploy to try and "keep me on the hook" by delaying responses and trying to refuse to cancel a service at a location I will no longer be at?

Regardless, the complete lack of response, zero feedback and statements that they will not cancel my month to month account is extremely worrying.

I feel like I am being significantly mistreated.