misinformed

I took a new data line with a laptop a few months back. At point of sale I was told I cannot change my plan to a top up plan, as my phone contract but the alternative will be to add a cap on my account that will suspend my service once I reach R199 out side of my plan, and I was OK with that. In November I receive a text message warning me about the limit and telling me that if I should continue I will be cut off, in December I went outside my data allowance and no text was sent and the same thing happened in December. In January I downloaded something on my laptop, I was not cut off nor a text sent. My bill I received in February for the charges in January came to R5087, which as never been this high. Why was I not cut off or notified. I was then told by a agent that the cap is not garenteed, why was I not told of this. But when I reached my limit in February i was sent a text and now I cannot use the service. The reason I asked for a top up or a cap line is to prevent this from happening and why was I sold a service that does not work. I dispute the bill, and I would like this to be investigated.