Overcharged on cancelled service

Being overcharged with a service that was initially cancelled 03.01.21 and have been battling to have this service cancelled since i bought this under MTN attention 20.04.21. I downgraded specifically because of previous problems. And still continue to being charged with 2GB recurring bundle additional to the 80GB that I actually signed up for. That is now 7 months worth of overcharging. That was supposed to be cancelled.