Poor service

I have a contract with MTN since 2023. I changed my banking details in February 2024 and banking details were sent to MTN. For the past 4 months I have been receiving sms’s saying I am owing and need to pay. Each time this happens I call MTN and inform them that they are deducting from the old account and then pay manually and share proof of payment. The service agent would say it has been sorted and I won’t have the same problem again. The following month the same thing would happen again and call them again. Every time I speak to a different agent and would go through the same issue again. The last agent I spoke to in June said this happens because there was no debit mandate and we did that and he promised that they would deduct what I owed for that month which was 1 month instalment. Guess what they deducted twice the amount. I had to reverse it and pay manually and send the proof of payment. They did acknowledge receipt of my email by the way of proof of payment. Now my service has been suspended, HOW!!!!!. I am done with MTN and wish to cancel this contract. Have never been so frustrated in my life.