1. Disable cellular app updates
Automatic updates of apps will always take a toll on your data allotment. So, make sure apps are updated only when you are on WiFi. Go to Settings > iTunes & App Stores and turn off Use Cellular Data.
2. Disable WiFi Assist
Wi-Fi Assist is a great feature where your iPhone hands off a weak Wi-Fi signal to your cellular network to prevent pages from loading slowly or not at all as it clings to the last remnants of a Wi-Fi signal. If you sit on the edge of a Wi-Fi network at work, say, then your cellular network may be assisting more than you’d like and running up data charges.
To disable Wi-Fi Assist, go to Settings > Cellular and scroll all the way to the bottom to turn off Wi-Fi Assist.
3. Disable background app refresh
iOS apps can update in the background, grabbing new content as they sit idle so they can show you the latest news when you return to them. Go to Settings > General > Background App Refresh and you can turn this setting off completely or go the a la carte route from the list below and choose which apps update in the background.
4. Disable iCloud Drive on cellular
If you use iCloud Drive to shuttle documents in and out of the cloud, those data transfers can add to your bill, especially over a cellular connection. You can restrict iCloud Drive to Wi-Fi only by going to Settings > iCloud > iCloud Drive and scrolling to the bottom and turning off Use Cellular Data.