MTN with wrong adverti*****t and bad customer service.

I took a contract with MTN in december for the laptop, router and data, which i received the laptop in January. Then the laptop is faulty since when i log in it take about 30 min to log and also i always have to restart it before i do something cause it will just freeze, so in february i get a call from one of the consultant from MTN for the upgrade which they gave me 40 gb monthly and i did explain to the guy i spoke too i only need for day and the guy said i can choose so he gave an false adverti*****t. that when i saw while using it that they split the data then i did give the a call again and i spoke with a lady which she checked for me and request to change the package which i had agreed on and she told me i will pay R522 this amount for cancellation fee and the instalment then we agreed on that to my surprise when i receive the sms that i have to pay R746.13 and i have called them they told me it for the pro rata the other amount which i was not told about and now the account is in arrears because i wont pay for something i did not know about and i was not told when they changed my product. can you please help regarding this cause now they making my credit report look bad as i did not agree and when i call them they put me on hold for long time then drop the call. i am really not happy about this service provider cause i even went to MTN branch regarding laptop to cancel and they stated that i need to pay R5500 for the cancellation. please help.