This is Apple's support document on it...
https://support.apple.com/en-us/HT204051#section_5
It's usually a last resort. Have you tried turning on airplane mode, waiting 30 seconds then turning it back off? This will cause your cellular radio to try to reacquire its signal. Also a simple hard restart, that is hold the power and home buttons at the same time for about 10 seconds until the Apple logo appears. This will cause the iPhone to reboot as we say in computer terms.
Resetting the network settings is NOT the same as factory resetting your phone, you will NOT lose everything, BUT you will lose all your saved WiFI networks in particular.