Cell C's administration

I have been using Cell C Wi-Fi for almost 4 years now. The connection is great, but the problem is the administration. I was charged for exceeding my bandwith in September 2019. This was impossible because I am on capped Wi-Fi. I have followed up this matter with emails, calls, and even visiting Cell C branches, yet this has not helped. The amount FROM LAST YEAR is still being deducted from the account even though it is falsified, and Cell C acknowledged their error, and promised to rectify it. All in all, the administration of Cell C fails. Dismally.