Limit iPad data when tethered to iPhone (iOS equivalent of "Trip Mode" or "Little Snitch")
A simple Web search revealed this app which claims to monitor data usage on your iOS device.


My Data Manager VPN Security


Some of the features as listed in the app description are:


Monitor your data usage on mobile, WiFi, and roaming
Find out which apps are eating up your mobile data
Get alerts before you reach your data limit to avoid overage fees


The app is available free of cost from the App Store.

Note: No affiliation whatsoever with the developer of the app. Based on the app description and ratings, it appears like a useful app.