MTN *******ly charging me for their error in cancelling my sim only contract early

I have been an MTN customer since 1994. I called retentions on 22 April 2022 to give 30 days notice (future dated cancellation) on my data contract. The consultant advised that when my 30 day notice expires I should change the number to PAYGo. I agreed to this, however instead of a future dated cancellation she processed it on the day to an early cancellation! And I have now been charged for cancelling early! I have called and emailed consistently since 22 April and nobody can resolve this. I am told a supervisor will call me but never does. Shocking service and as soon as my other 2 contracts end I will be changing service providers after supporting mtn for 28 years!!!