Feeling Helpless

good day i had taken out a contract for internet services with telkom mobile on the 25th July 2015. I took the modem home set it up and the reception was extremely poor. i went back to the store on the 27th as the store was closed on the 26th.the guy checked the modem and said everything was fine the reception in the house is a problem.The solution was to cancel this contract and get an adsl line fitted. so i gave back the modem and sim card,he told me it wouldn't be an issue as it falls within the 7 day period,and would be canceled within 21 working days .since then i have been charged pro rate and 2 installments for a modem i don't have and never used . i have tried the call centers and supervisors,everyone gives me different stories some say the right department will call within 72 working hours and up till this day that call has not come. i feel ****** and helpless in this situation. kindly assist me.