MTN keeps debiting wrong bank account 😡

I have a month to month contract with MTN. In the beginning of November 2024 i went to the MTN store in Secunda to update my banking details as i have changed banks. On the 4th of December the old account was debited resulting in me incurring bank charges for a returned debit order. MTN also charged me for a failed debit order. I went to the Secunda store and reported this matter. I was told they cannot assist me with anything, i must send an email to their complaints mailbox. All they wanted was for me to pay what was due so they can lift the suspension on my account. I sent the email but all i got was confirmation from MTN that they have my new banking details on record. They failed to explain why my old bank account was still debited. Today, the 4th of January MTN again attempted to debit the subscription from my old bank account! What must i do to be assisted? Should i keep incurring unnecessary costs because of their mistake?