MTN milking you for extra money when your contract ends and you don't upgrade.

I ended up paying 37 installments on a device instead of 36 as per contract and customer care says there is nothing they can do. I gave notice to cancel a contract on 17.11.2023 as the contract ended on 16.12.2023. I saw that I still was billed for the device up to 01.02.2024 debit order thus paying for the device from 01.02.2021 up to 01.02.2024 counting 37 installments not 36 as per the original signed contract.