See these Apple articles below:
If you see No Service, Searching, or SOS on your iPhone or iPad
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your network provider settings manually on your iPhone or iPad
Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)
If you can't make or receive calls on your iPhone
If you can't make or receive calls on your iPhone - Apple Support
If you can't send or receive messages on your iPhone, iPad, or iPod touch
If you can't send or receive messages on your iPhone or iPad - Apple Support
Charge the battery > 70%.