To help reduce data usage, try connecting your phone, tablet or laptop to an available WiFi network when overseas.
Many hotels, restaurants and airports offer free WiFi, so be on the lookout.
Remember to switch off mobile data while connecting to WiFi.
WiFi Assist
WiFi Assist is a feature in iOS that automatically switches you from WiFi to cellular data if that connection is stronger. When this occurs, you'll be charged for the data usage as per your roaming selection.
Note: WiFi Assist is automatically turned on.
To turn WiFi Assist off go to Settings > Mobile > WiFi Assist.
Samsung Adaptive WiFi
Previously known as Smart Network switching, enabling the Adaptive WiFi allows your phone to automatically switch between WiFi and mobile data to maintain a stable network connection.
Check out how to disable it here.