BAD CELLULAR NETWORK SERVICES

I recently did an upgrade with Mtn. 2 Months after the upgrade i got a bill of almost R10 000.00. I enquired about it and they informed me that I did an early upgrade. I explained to all the consultants that assisted me that I was never made aware of an early upgrade and if I had been informed I would have never went to do the upgrade because I couldn't afford to pay that kind of a bill. After a month of back and forth with Mtn and them not understanding I decided to just pay it because I was scared it would affect my credit score. I made the payment on 26 July 2022. I am currently still being billed for the previous device that I have paid in full. I have been in constant contact with my local Mtn store consultant and it seems he is also not able to get things sorted. Mtn has cut my line for almost a month now and I can't make use of any services on my phone. Can you please assist me to resolve this matter asap because I have been waiting long and no one at Mtn's billing department is making an effort to sort this out.