Being Billed after the contract has been cancelled on expiry

My contract with MTN was ending on 11-08-2024, on 27-07 2024 I wrote an email telling them I won’t be upgrading, I want the contract to be cancelled on expiry. 02-07 2024 I got an email from MTN telling me to call a day after expiry and request my number to be on prepaid. 12-08-2024 received an email telling me I’m on month-month paying R224.38. On 13-08-13 I asked to be converted to pre-paid because I previously asked for cancellation of contract after expiry. On the 14-08-2024 I was converted to pre-paid. 21-08-2024 I got a bill from MTN of R848, for the month of 12-08-204- 11-09-2024. How are they billing me for the cancelled, contract?I did not get any services from them, I am on pre-paid. I’m not even getting an assistance from the with regards to this issue.