Hi there
This is an all too common issue with iPhones - they use a heck of a lot of data without you being aware of it. One of the issue is that if WiFi disconnects for even a split second, your phone will connect to mobile date and chew through that even though you are showing as connected to WiFi.
There are a number of things you can do about this. The first is to turn off mobile data to prvent the handset from connecting to data unless you need it. Secondly, download a data monitoring app such as Onavo and use this to work out what is using up the data. Then, you can remove that up or change it's settings so it doesn't sync too often. In fact, it would be an idea to change all your app settings to decrease how often they update.
If you have Facebook installed, you may want to go into the app and change the settings to prevent it from aut playing videos. Ever since Facebook enabled this feature, it has been using a lot more data than it did previously. Also, check to see if you have iCloud is enabled - it is worth disabling it as it can chew through data.
