Mtn is *****ing money from my account

Mtn overcharged me for 2 months, I logged a complaint with the relevant department on the 15 June till today I have never received any response only a complaint reference number from a no reply email ***. In May and June I have been charged R489 instead of R279 and no one can explain why, I have a wifi contract for the last 9 months and all along the correct amount was being deducted, when I have exhausted the given date my balance becomes zero and I am unable to connect, I am not on a Top up contract when the data is finished I can't connect. Can someone in Mtn just assist cause I reversed the debit order and will keep on doing that until I get an explanation why I'm being all of a sudden overcharged