iPhone have multiple solutions that can help to prevent going over data plan limits.

Low data mode

Turning Low Data Mode on will disable automatic updates, background app refresh and photos/videos/files won’t sync to the iCloud until you connect to WiFi.

1. Go into Settings
2. Tap on Cellular
3. Tap on Cellular Data Options
4. Turn on Low Data Mode

Turning off data access for specific apps

You can also limit data usage by turning off data access for specific apps. You can do this by following the steps below: 

1. Go into Settings
2. Tap on Cellular
3. Scroll down and turn off the toggle switches for the apps you don't want to use data

Turning off WiFi Assist and iCloud Drive

Additional features on newer iPhones like WiFi Assist and iCloud Drive are designed to give the user the best experience possible. However, these applications tend to use more data than ever before.  You can preserve your data by turning WiFi Assist Off and iCloud Drive by following the steps below: 

1. Go into Settings
2. Tap on Cellular
3. Turn off the toggle switched beside WiFi Assist and iCloud Drive

Background App Refresh

Background App Refresh is a feature that allows apps to update their content from the internet, even while you're not using them. To avoid using cellular data with this feature, follow the steps below:

1. Go into Settings
2. Tap on General
3. Tap on Background App Refresh
4. Tap on Background App Refresh at the top.
5. Select Wi-Fi 

You can also turn off the autoplay function for videos in social media apps.  If you are trying to save data for a specific app but unsure where to find the option do a Google search for “How to save data for <<app name>>”