Overcharged every single month!

I am honestly shocked by the poor service I am receiving from C Fibre. I was changed over from an existing fibre provider, Greencom, to Cell C. A big fuss was made about promising that our monthly subscription would remain as is, at R1200 per month for 100Mbps uncapped fibre. However, from the very moment that I was switched over, my bill was somehow changed to R1399 a month. For four months I have been complaining about this, and even received assurance from a woman named Lyn that this was rectified, and lo and behold, I was just overcharged YET AGAIN. I do not know what to do anymore, I am being ****** every single month and I can't even ask the customer service department to help me because they just flat out lie about it.