MTN billing me for services I have cancelled in June 2024

I was using their home portable wi-fi. I took the device back to their Mall of the North (Polokwane) branch on 14 June 2024. I was told that they would deduct the last amount end of July 2024, which they did. Now, out of the blue, end of August they wanted to debit my account, I told them how can I be billed for a device I returned to the store and cancelled the subscription? Now they debited my account, and I reversed the debit and send them an e-mail reminding them that I am no longer their client.