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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 the following Apple articles:
Update your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch - Apple Support (IN)
If your iPhone, iPad or iPod touch won’t update
If your iPhone, iPad, or iPod touch won't update - Apple Support
You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC
Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)
If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder
Update your iPhone, iPad, or iPod touch with the Finder - Apple Support
You need about 7 GB device storage and good Wi-Fi.
If they are On, turn Off Hotspot & VPN.