MTN continues to debit me for a Contracted that end in April 2020

Just summarising....

My MTN contract ended by end of April, and I asked them on 28 April to migrate it tom Prepaid. It seems they delayed this migration due to COVID-19 or something - should not be my fault as I reported the migration on time. MTN only did the migration on 19 May.

They billed me (debited me on 1 May - which I thought was ok). Then they billed me again on 1 June! I reversed this bill! The contract should have been migrated to Prepaid as of 31 May latest (they decided to do this on 19 May - and I am not going to let them bill me for their mistake)!

July they billed me once again - and they debited twice! Unfortunately, I missed this one, as I thought the matter was long concluded.

They also billed me in August (again I missed this) and September - I saw this and I reached out to them also trying to address the previous 2 months (July and August) and no one can assist me at MTN. They confirm that the number is on Prepaid yet can't explain why I am being billed!

Now in September, they billed me once again!!! I just don't know what else to do!