BREACH OF CUSTOMER RIGHTS BY MONDO AND MTN

I open a device contract last year with Mtn last year November through Mondo. The device I received I was not happy with it and because I was still within my cooling off period i immediately returned the device back. I called the agents at Mondo and they told me to return the device at the store and I did so. I even Recieved an sms confirmation that the device has been picked up by the courier and being sent to the warehouse. At end of the month of november I find myself being debited for the same contract I cancelled and in December I get a sms saying they have sent my monthly invoice. WHAT INVOICE? I called in today to follow up and the agent confirmed that the account is Very much ACTIVE. Why Am I being ****** by mtn and mondo? I cancelled the contract within my cooling off period because I was not satisfied with the device and that is within my rights according to the CPA of South Africa. I NEED QUICK RESPONSES FROM MTN OR MONDO. because I actually want to take out a deal that I WANT but I will not Risk to be double charged for something I CLEARLY CANCELLED. Oh and I Don’t even want to start with the horrible customer service of Long queues you have to wait to get ahold of a customer. MTN doesn’t respond on any social media platforms. IF MONDO IS A SCAN LIKE PEOPLE SAY IT IS THEN IM DEFINITELY NOT GOING TO TAKE ANY DEALS THROUGH THEM NOR MTN.