Contract cancellation

I have been trying to Cancel a contract that I made with MTN through Mondo since December 2021 till date, because no one, I made my CANCELATION within 10 days after receiving the device and since then MTN has been billing me for a device that was taken from me by DSV back to them, because no one is able to help me I was forced to reverse the all the money they kept deducting, now my problem is that I had an existing data contract and now it also has been suspended due to the money that I owe, I can't pay for the data contract without paying for the device I don't have ,MONDO can't help and MTN cannot help.WHAT DO I DO?