First of all, please make sure you follow our SIM set-up guide and install an APN profile in your phone.
Secondly, try the following troubleshooting:
- Double check if the APN profile is successfully installed (Settings > General > Profile)
- Turn Airplane mode on and off
- Reset network settings (Settings > General > Reset > Reset Network Settings)
If you reset the network settings, you need to install an APN profile again.