Mtn- home Internet billing issues

I recently got an mtn home internet contract I was told to pay 2 months upfront ..I received my first invoice after paying it only to find out the first payment of 2 months was not allocated and hence now I need to pay interest plus triple the amount again? I’ve called numerous times I’ve went into the store and they said they can’t send me my invoice neither can they view it ?? This morning my service is was suspended ? Please can I ask for better assistance as this is unacceptable