Unfair treatment by MTN South Africa

I have bought a cell phone on a 24 months contract at Brooklyn MTN store in Pretoria. After about 8 months of paying R642.18 which is what I signed for, the monthly payment has changed to R730.00. I have sent complain to Mtn including writing them an email. They said I should sent them a contract, I then went to Brooklyn Mtn branch where I bought the cell phone from on Saturday 15 February 2020 requesting for a contract, I was told it will be sent to me through email address. Mr Ligege AS ***