Wifi data disappearing

Used to have ADSL internet with 10gb data. Switched to LTE Wifi via a sales agent for Telkom. 10gb data disappearing in about 7 days since 1/12/2015. On 5/12/2015 bought another 10gb. Same depleted within a week. Bought another 5gb and again same depleted within days. 25gigs within a period of two weeks. We never used more then 10gigs in all previous times even when kids download movies or watch You Tube. January data also disappeared within a week. Discussed with Telkom at their shop in Menlyn. They were unable to help or solve the technical query. Bought another 5gigs on 15/1/2016 just to check. All devices were delinked from wifi and router switched off for most of the time. When checking data balances it was clear that data is leaking somewhere but Telkom cannot tell me whereto or give me printout of usage or which devices used the data. Not prepared to continue with 2 year contract for which I cannot use wifi, paying for data and all data leaking out. I want my contract to be cancelled with immediate effect.