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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, Searching, or SOS on your iPhone or iPad - Apple Support
Charge the battery > 70%.
See also -> If you can't connect to the App Store, iTunes Store, or other Apple services
If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support
-> If you tried all of the above steps and you still can’t connect, contact Apple Support.