MTN Home internet- Charged for an extra month and will not investigate or correct it

I took MTN Home Internet 5g. I paid for December 2023 and January 2024 on 6 December 2023. I was supposed to be debited in February only. MTN debited me in January 2024 incorrectly. They insist that they did not charge me incorrectly but they did and they now charged me for an extra month and will not investigate or correct this.