Cell C pro-rata abuse

Got a 50gb contract 4 days before month end. They gave me 6gb which is about 12% of the 50gb. Sim card was not working couldn't access the 6gb. The month ends still sim is not working went to a Cell C store to get a sim swap done. Now its time for me to pay their pro rata is almost equal to the instalment. Pro rata for data I never used on top of that charging me double instalment for 12% data received in the pro rata period. Please refund me my 88% of the pro rata.