Paying for contract and not receiving data

Got a mifi data contract with MTN.. Firstly it was a shlep getting the deal signed as they had multiple issues with their system. The first moth went fine. The second month about 10 day after the data was loaded, a message kept on popping up. It said that the data was depleted and I could not gain access to any data. I assumed that one of my devices updated without me knowing. However when I received my invoice the following month, it indicated that I only used half of my contract amount. At this stage again the message was popping up saying I have no data left even though it was only loaded approximately 10 days prior. I went to the shop were I signed the contract. They said they can not help as it shows that I used all my data on the sim card. So I said well if you cannot help me this month (before I get the invoice), what about last month. An hour latter...They said they could not help me but will log n tickets with head office. I then stated as they did not deliver on their side of the contract I would like to cancel the contract. They stated I can not do that and someone will give me a call. It is 7days later... I am still waiting for that call.