Dissatisfaction with a cellphone contract which impacted my credit score.

I took a cellphone contract with MTN for two years from 2021 April to 2023 May. The contact ended however my account was still debited for two more months. I made enquiries and I was told that I didn't "inform" MTN that I was done with my contract. I did and the account then stopped debiting however 7 months later there was another debit order which I then reversed. This then affected my excellent credit score as it reflected a missed payment. I missed out on great deals as a result of my credit score which was unfairly impacted by MTN.