MTN awkward debit date

Upon on upgrading my cellphone contract with MTN in November of 2020, my debit date was changed from the last day of the month to the 12th of every month without my consent.

I have called MTN & have also gone to the nearest MTN branches to have the debit date restored to the last day of the month (upon on which I was told that the debit date is set to the last day of the month, but the agent changed it again anyway). May I please have this resolved ASAP as it's affecting my natural spending & also runs the risk of being forgotten as I move my funds around different accounts at times.