Incorrect invoice/bill

I have had a horrible service with MTN/Mondo partnership. This started when I ordered a Sim card online for data and I was given the wrong package. I then canceled within the cooling off period however I am being billed R1700 by Mondo and MTN when I do not have the items. I have reached out to Mondo and MTN and have been made to go back and forth with no real resolution. I am now at a point of frustration as this has impacted by credit profile negatively and no one is willing to assist me. The only active device I have with MTN is a HP Notebook and Router that I can account for but all these other bills, I have no idea where they come from. I haven't been satisfied with the customer service from both MTN and Mondo as I have been sent back and forth and at the end of the day I'm the one who is an arrears for items that I don't have. I really need someone to look into this as this is extremely frustrating when I ensured that I have no bad credit on my name from both companies and someone needs to have this bill removed under my profile.