5 Reset Your Settings
On the simplest level, head to Settings and flick Airplane Mode on and off. Or turn Airplane mode on and off from your iPhone’s Control Center. This will force your phone to try reconnecting to find a signal, and may solve your issues.
Double check to see if you have any operating system updates to make – this can also help clear up any bugs or problems that may have been interfering with your signal.
You can also try resetting your network settings (on iPhones, you can find this option in Settings > General > Reset > Reset Network Settings), although this may take at least a few minutes to reconnect.