C Fibre incorrect billing

I have been contacted by Cell C Fibre in October and signed up for a C Fibre package. I was told by the C Fibre consultant that I would need to pay an upfront fee of R856.00 which I did. My installation and line connection was only done on the 13/11/2018. I have now received an invoice for R1369.60 which is incorrect as I have paid the monthly fee upfront. I have spoken to C Fibre agent Lindo Zwane who said that the statement is incorrect but cannot issue me with the correct statement. His supervisor Neville Moodley refuses to speak to me as they say that there is nothing that they can do. I have requested they send me a new statement which they say they cannot. I am really disappointed with the service and see that Cell C is ripping people off.