Price increase 5 months into 24-month contract

I signed up for a 24-month contract stating R199 pm and it's increased to R209 pm 5 months into the contract. that's unacceptable! What's stopping you from increasing it many more times in the remaining 19 months of the contract, until it's unaffordable for me and I can't get out of the contract, but you can increase prices? I signed up for R199 for a reason and that was how it was advertised - contract for 24 months. Changing it now is not acceptable!