Mtn and their “brilliant” customer service

Truly terrible service from Mtn. I took out a WiFi contract with them and everything went very well for the first month or two. I had to change bank accounts and I notified them but apparently they did nothing and as a result my account went into arrear. I tried calling and going in store to cancel but it’s always the same story. They can’t help me cancel it I need my sim. I even took my sim with but still the same old story. I even followed all instructions to do what I have to but they blocked the sim so it’s useless. I reached out to head office and the dept department many times but NO ONE comes back to me. I literally just want to close my account because it’s leaving a bad mark on my credit score. Can anyone please help me?