MTN has Breached my Contract

I am not sure if this is the right place to lay my complains. I had a contract with MTN for R200per month on data for 12month, before the end of the contract I took another contract of an OPPO A53 smart phone for R270per month. The total installment of these contract was R470 per month, which I started paying on April. Then MTN decided to change the monthly installment without my concern to R1039per month, from the month end of Jun which I ended paying on the 15th of Jun since the I have been trying to find the solution to this because it is disturbing my budget and now it is putting a stein on my Credit record. I'm in need of a solution I can't take this anymore. please some help out