Overbilling and No Transparency on the Product Offered

I upgraded my wifi contract with Cell C in Sep 2017 of which I was told that I was going to pay R199 for 20GB anytime data and 20GB at night, the contract kicked in on the 21st of Oct 2017 and now when I received the bill, it is completely different from what I signed up for and on top of that it is an open line and I am charged R998.93 for the month of November. When I upgraded I was promised that I will be on the same contract I was on before upgrade which was 20GB at nite, 20GB anytime but now I will be charged R199 instead of R399 I was previously charged. I am so disappointed at Cell C consultants, they would do anything to sell the products even if it means lying to the customers and on top of that they have a nerve to tell me that I have to pay an amount of R958 to cancel the contract. First of all this is not my fault and I am over charged for something that I never agreed to and now I am asked to pay this cancellation fee as if it's my fault that I want to cancel. Please Cell C own up to your mistake and don't you dare ever charge my bank account of any amount again concerning to this. I need this account closed as soon as possible and you can come get your wifi router.This is really disappointing and I have been loyal Cell C customer for very long!