MTN unauthorised overcharging on data contract

Since the beginning of my data contract in 2018 MTN has been overcharging my account on random occasions. After a lot of back and forth they told me that they were charging me at prepaid rates instead of the amount I signed for and agreed upon in my contract.

My contract has come to an end in September 2020 and they have charged me 4x my contract amount. I have been calling the call centre and have received multiple reference numbers but no resolution. On top of that, it seems that they have blocked my data so that I can't use any of it. They also want to deplete my data, THAT I HAVE PAID FOR, when I move over to prepaid. But I'm unable to use it.

Their staff doesn't seem to know what they're doing and lately when I call I just wait to be transferred to a different department or the system is offline.

Who do I need to speak in order to resolve this????