MTN pathetic in resolving over billing issue for months!

I renewed my contact with MTN in October 2020. I have been a client of MTN for more than 10 years. Since my upgrade in October, I have been charge almost double the amount I should be paying monthly. I have been to the Somerset west store (where I did my upgrade) several times. The staff there have been very helpful and they verified that I was incorrectly billed. They managed to get my billing correct somewhat but I am still being charged too much. The fact that I have been speaking to the store staff for since January 2021 and they verified that MTN is in the wrong, my billing is yet to be fully corrected and I have not received a credit on my account or any money back! I cant even indicate how much MTN owes me because I do not know what my correct billing should be??? I do know that it is easily in the thousands!! This is pathetic service MTN, how complicated could it be to do a simple credit or refund for money that you took erroneously??? Please fix this MTN, this is not rocket science, I am one so I would know!!!!!!