How do I monitor or limit my mobile data usage?

Go to Mobile Network > Statistics to view Data Usage Total, 1 Month, and 1 Day.
Set a limit in Max Data Usage (MB or GB) and enable Traffic Limit Notification Switch with an alert threshold (default: 80%).
Set Traffic Overrun Action to Disconnect to automatically cut off mobile data when the limit is reached.

Tips: To disable roaming abroad: Mobile Network > Mobile Connection > Data Roaming > Disable.