To troubleshoot why your phone may not be connecting to a mobile network, try these steps:

1. Restart your phone by powering it off for a few seconds, then turning it back on.
2. Check that SIM Card and SIM tray are properly seated.
3. Make sure mobile data is enabled in your network settings.
4. Check to see if you are within coverage of a particular network.
5. Reset network settings on your device.