MTN not fixing their mistakes

I upgraded my account on the 4th of August 2020. Because I am a pensioner, I took a cheaper contract of R399. Without my consent, MTN gave me an extra 2GB on top of my contract and I am now paying R659. I have sent them a message, I have phoned them, but with no avail. They keep on suspending my account and when I phone them, they keep on telling met that they will fix the mistake. My reference is ***