MTN unscrupulsly adding amounts to your contract fee

Have 3 x 24 month contracts with MTN. Everythings fine until February 2020 about 14 months into the contract. They added 100.00 extra on two of the accounts and not on the other for no reason. Called a million times to fix this and still no response. No credit in sight. The latest account still has the additional 100 per month on the 2 contracts. Insanity. Where do i turn to fix this please?