Debit order not canceled

You I want to file a complaint against MTN. I talked to several people at their call centre (after I was transferred from one to the other and hold for countless minutes!) without any luck to cancel my contract. Eventually someone helped me on the messenger app after I said that I am now going to complain on hello peter. So they told me my contract is now made prepaid - which means no more debit orders - and I thanked them for their help!! Tonight again, the debit order went through my bank account. I am struggling with this from June 2020. I also asked my bank to stop the debit order but they said MTN has to cancel it. How am I going to stop this?