Price Fixing

I subscribed for 28 days unlimited data with Vumatel for R394 but their system doesn't accept R394, and rather R395 whichbI have been paying nevertheless. My concern is their price fixing as they only supply this unlimited wifi data for between 22 - 26 days per month instead of the agreed 28days. The latest example is having paid on the 29 Aug 2022 and they switched my data off on the 23rd Sep 2022, which is 26 days. Someone pls help me get the service agreement I was promised.