Please help I've tried to call without anyone helping me

I Signed A contract with mtn I wanted a data simcard that was R199 per month that what I've agreed on but now they've sent me my statement that says I owe R705. 66 and I was discounted I got the simcard on the 26.02.2021 and they said I will start paying at the end of much and when I called them telling them that I want to cancel the simcard because I feel ****** all those pro ratas they talking about I'm was not told about and I get it maybe the first month it can be more than expected with a period of less than a month I was charged twice at first I was given 20gb before it was finished they loaded another data and my simcard was suspended I went to mtn wanting to know what was wrong then the guy from mtn said it might be the fact that I didn't pay but I said to him the sim was still new and my agreement with mtn was that I will start paying at the end of march then he helped me so when the statement was sent to me I've realised that it was not our agreement so they must cancel the simcard I went to mtn again theyve said I must call customer care as they can't help me with that I've called last week almost the whole week with no luck and on Friday they've they have a system problem on which on Thursday I was on the line almost 2 hour when they transferred my call still I was not assisted.... I want mtn to rectify their mistake or close the simcard that is on my account as I will not pay more than R199 that I agree on... They should have explained everything to me before I agreed on anything to me that was a false adverti*****t I will not pay more than what I agreed on..... Nd why would they say my simcard its an old system yet I'm having a new simcard with me and one of the consultant said to me actual this R705. 66 was discounted on which I'm still confused on how it was discounted because I agreed on paying R199 not more than that please help me what my I do to pay the R199 that was advertised to me or cancel the simcard