To set mobile data limit, tap Set cellular data limit.
Drag the Limit bar up or down to the Desired data limit. Drag the Warning bar up or down to the Desired warning data limit. Note: When the mobile data limit is active and has been reached, the mobile data block will remain in effect until mobile data is manually re-enabled on the device, the limit threshold is changed to a higher value, or a new data usage cycle begins. If you set data usage limits on your device, they do not communicate with AT&T systems. You will still receive AT&T system generated notifications as you approach your data limits.
To view data usage by app, scroll down. Note: To restrict apps from using data while running in the background, tap the Desired app > Restrict app background > OK.