Poor service and incompetence

I returned the cellphone device with 10 days of receiving it from Mondo. I requested the contract to be cancelled and few weeks later i got the confirmation that the contract was successfully cancelled but to my surprise I was neing debited every months.

It has been 2 years now going back and forth asking them to properly fix this issue and nothing is been done.

Now I am getting threats from the debt collectors to blacklist me for owning MTN since Mondo **** about cancelling my contract.

Can someone assist me please