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Try first to optimize the range of the mobile network, see this Apple article:
If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
See these Apple articles below:
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, iPad, or iPod touch - Apple Support
Low Power Mode is On.
Charge the iPhone & wait until the battery is fully charged (100%).
The Low Power Mode will turned off automatically.