In the case of 4G not working on iPhone, it could be an issue with the service provider – probably the cellular networks bands aren’t compatible with your iPhone, just in case the operators in your region aren’t following standards which is very rare. Otherwise, it’s probably a glitch on your iPhone and few trial and error methods could get the iPhone get connected to 4G network again.
1. Check Cellular Connection
2. Turn Airplane Mode On & Off
3. Remove the SIM Card and Reboot your iPhone
4. Is the SIM PIN Turned On? – Switch it Off
5. Reset Network Settings
At least one of the methods described above should solve the problem. If not, you should pay a visit to the nearest authorized Apple service center.