Unhappy customers

I took a contract iPhone 11 which I'd 36 months the agreed amount was 750, 6 months later received a call from one of their agent offering me extra days I refused and she said it was free they offering it as a token of appreciation as their client, but month they debit 850 when I inquired it was for the axtra data I went cancel the extra data it would be canceled in few day but it was never canceled I've been gooling to their with no help .this is not what we agreed on