I logged a cancellation on 30th January with MTN Brooklyn

I logged a cancellation on 30th January with MTN Brooklyn. I returned router on the same day. This was a month to month package and I cancelled within the month I already paid for. They tried to debit me every month since I cancelled. Now this has affected my credit score. I went to the shop to enquire and they say my account is with legal and that I need to pay for 1 more month. I told them that will never happen as I cancelled on time and returned their device on time. If the store didn't capture the cancellation on time,that is NOT my problem. Just to reiterate,I will NEVER pay for a service I did not use.