CHARGED CANCELLATION FEE & RE-BILLED - FOR NON-WORKING MODEM RETURNED!!!!!!

Took a Contract out for a wi-fi Modem and had to return same for a credit as it DID NOT WORK in the Underberg area!! NO SIGNAL WHATSOEVER!!!!!!! Was told in Store (by a Hillcrest Agent), that it would work (big fail on the part of Cell C)!!!! Was informed by Cell C (Hillcrest), that the amount for the returned Modem would be credited to my account between 1 - 7th August. I have since received my latest Billing for August and have been charged, yes charged, a CANCELLATION FEE for the returned Modem!!!! SO NO CREDIT FOR THE MODEM RETURNED, BUT CHARGED R832,50 CANCELLATION FEE!!! Please..........do Cell C actually know what they are doing? From a VERY disgruntled Customer!!!