Updating

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

Check also the Wi-Fi network, see the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - 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.

Then do the update as in Kurtosis12 response.

You need to update your carrier settings.

Manually update your carrier settings on your iPhone or iPad - Apple Support

Carrier settings are pushed from the Carriers. You may need to contact your carrier with your IMEI number to confirm they sent an update or if an update is even offered. If they have not pushed an update, the manual method will not work.

Look at my previous post, and try to solve those issues first.