To investigate rising mobile data usage, first match the period shown on your phone with your carrier's billing period. Then inspect individual apps. A total without matching dates is not a useful diagnosis.
Check the correct line and reporting period
An iPhone lists cellular use by app and allows cellular access to be disabled for individual apps. Apple also provides a statistics reset option and directs users to their carrier for the most accurate current-period usage. Record the current figures before resetting them. Use cellular data on your iPhone or iPad
Pixel devices provide an app-data breakdown as well as configurable warnings and limits. Google cautions that carrier measurements can differ from the phone's figures. Menu wording may vary on other Android devices. Reduce and manage mobile data usage
Identify the behavior to change
Make a list of the largest users and what you actually need from each while away from Wi-Fi. Check the app's own download and media preferences before restricting it entirely. For example, decide whether a streaming app needs mobile access for listening, downloading, both, or neither.
Keep navigation, travel, and communication needs in view. After changing a permission, try the app away from Wi-Fi while it is safe to do so. Do not discover during an important trip that the setting you changed prevents a needed task.
Use alerts as a review point
A warning is a prompt to investigate; a limit is a deliberate intervention. Read the device's explanation of what happens when the limit is reached. Keep the carrier's account information available so unexpected charges or measurement disagreements can be investigated with the provider.
The aim is a controlled reduction in unnecessary use, not a promise that the phone's counter will exactly match a bill. Review another comparable period before making further changes.