In breech of contract

I app**** for a contact in November 2020, got a Huawei p40 lite from MTN on their black Friday special for R550 a month including data and a smart watch. Since having this phone I have been having endless issues with MTN double debiting me more than double the amount. Last month they took R703, tried again for R720 which bounced, this month again I've had a debit order going off my account for R1261. HOW IS THIS POSSIBLE?? The contract I signed is for R550 a month all inclusive. I want to stop the debit order and I will go in every month to pay what I agreed to pay. Please contact me urgently ***