Vodacom is unfair and unproffesional

I took a cellphone contract with Vodacom in June 2019, I had defaulted payments due to being affected by the pandemic. In August 2021 I paid the arrears and the premature cancellation penalty but then Vodacom delayed the cancellation until they loaded airtime on 1st September 2021. Same day later they sent notification the contract has been cancelled and they took back the airtime and converted the SIM cards to prepaid as requested. Now it's October I got an invoice and a call that I owe them for September because I didn't include it in my cancellation penalty and my contract has not been cancelled. When I call them I can't be assisted because the number I am inquiring is a prepaid but they can call me to say that my contract is not cancelled. What should I do with this matter, is it really my fault that they couldn't process my cancellation in time? I wouldn't recommend this company to anyone, not even to a dog, it's a traumer. Even their complaints department is useless, you won't get help.