Cancelation of a expired contract such a disaster

I Canceled my contract on 25 October 2023 as the contract expired. I received the cancellation reference *** on 30 October 2023 and was advised that it would be a prepaid sim now. All of a sudden I receive an invoice for R1800. I used to pay R320p/m. Where does MTN come up with this? No response from the Retention department. E-mailed and phone. I cannot stand MTN.