MTN CHARGING DIFFERENT AMOUNT EACH MONTH

I signed a data contract with MTN for a wifi modem. The advert and the contract specifies that it will only cost me R239 a month. Excluding all the problems I have experienced getting this modem and being debited for something I didn't have. Taking a month to be delivered and the cell nr being deactivated and struggling to get the sim card activated. I am now having an issue where I am being charged different amounts each month. In June I got charge R264.87 ( this has been refunded), July I got charged R432.36. This was explained to me to be "prorata data". I was then promised I will be charged the correct amount from this day on. To my surprise this morning here I get charged R311.71. MTN I want an explanation, why aren't you following a legal contract? I did not budget for an amount to come off my account as a surprise every month. I have had cellphone contracts in the past and I was charged the same amount each month.