Billed for a No-service month

I called Vox in September to request a SIM only Data deal which I saw advertised. At the time I had been using my Router for prepaid data using other service providers including MTN. I was under the impression that getting a Data only deal would be easier. When the SIM Card was delivered after a few days, I called the technical team and the first guy couldn't assist. The next day the second guy informed me that my Router was not compatible. I was adviced to cancel the service and was informed that there will be no charge for the cancellation. I acquired a compatible Router in November and called again. This time the process was successful. The Problem is that Vox sent me an invoice for the month of September when there was No service. I returned the debit order and have been calling and writing emails to Vox and there was no reasonable assistance. One lady told me that Vox bill in advance and that I have to pay. I find it Unreasonable to pay for service that did not exist. Vox please assist me. I still want to be your client but I cannot pay for a month where there was no service.