Billed for a cancelled contract

In February 2022, one of Mondos' agents called me, offering me a portable router deal but instead of receiving a portable router the home router was delivered to me. I then called them to recollect the router and I cancelled the contract within 6 days cooling off period and I also received an email that said I am not liable to pay any cancellation fee since the contract was cancelled during the cooling off period. But to my surprise every month I am receiving a bill of R3,125.12 or above and they keep debiting my bank account. I have been calling Mondo's and MTN and they always give me references but I cant get help. Is been 7 months but no one is able to assist.