Update + Reset Your iPhone
While we covered carrier settings updates earlier in this list, there’s also a good chance that an iOS update is causing problems for your signal. It’s happened in the past, so we always recommend downloading the latest version of iOS available from Settings > General > Software Update.
A more “scorched earth” method of fixing cellular problems is to reset your Network Settings. This can fix a wide range of bugs with cellular, Wi-Fi, and Bluetooth, but it does mean you’ll need to reconnect to all of your Bluetooth accessories and Wi-Fi networks. If you’d like to proceed, head to Settings > General > Reset and tap Reset Network Settings.