********** Data and overpriced

I currently have contract for Data MTN, my upgrade was due in August 2019, which I apply for via MTN app when I was notified, better yet it was a promotion where u get Mega40 for R199, I did not hesitate, I apply, the following day they sent me a Sim card via courier, September first debit order after upgrade I'm charged R697.50, I thought it's prorata since I app**** after receiving my usual data. October, November and December same deduction R697,50, now I'm totally not understanding why so much while I'm expecting R199 only for deduction. I contacted MTN the call centre agent got my frustration, by January 2020 no deduction happened since MTN owes me, just after my concern with the amount I'm being charged, the Data reduced to 10gig anytime 20 gig night, I contacted MTN again for clarity and complaining about data, I was told according to my contract I am on My5gig plan that is why I am getting on 5gig reccuring + 5gig internet bundle, as for night shift gig I'm not even paying for it, but I did not apply for 10gig, and I did not get the agreement documents as I kept on being asked about it by call centre agents on my complaint. I have been given the number to contact which deals with direct marketing because I apply via app, until today no feedback or corrected Data. Why is MTN treating me this way, while I have another contract for cellphone which is due for upgrade I am now sceptical to even renew my other contract, they are shady network vs boza network