Incorrect billing

I had downgraded my fibre package in October 2021 on the online portal from the R529 to the R399 package. Up until January I have been still been debited each month R529. According to one agent there was no downgrade that took place. However, the portal clearly states something different, then another agent states there is a downgrade however back office hasn't processed it. I cant understand how one can downgrade your package online but still get charged incorrectly & receive a slower service. And still not get a refund for the incorrect billing. Oh & the whatsapp service is so useless they might as well not have that service at all. Having to wait a day to be transferred to an agent is shocking. The service sucks big time. I have all screenshots of how the downgrade took place yet I'm being over charged.