Finding iPhone Data Usage and how to Reset
This video shows how to find Data Usage in your iOS device, if it's iOS 9 or higher.
Older versions, go to Settings, General, Usage, and then Cellular Usage.
Also shows how to find WiFi Assist and iCloud Drive, both of which should be turned off to save data usage.
Lastly, shows how to reset the statistics so you know consistently how your data's being used.