If you can't connect to a cellular network on your iPhone, make sure that cellular data is on in Settings > Cellular*. On your iPad, go to Settings > Cellular Data. Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before. Try updating your phone. If still not working, it look like water damage. Even you think there is no water damage, note that only small drop of rain water can enter in the device and make corosion. But anyway, i would suggest to take a look to SIM tray as last.