Update software
How to fix password to update software
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(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode
If you forgot your iPhone passcode - Apple Support
(iOS 15.2 and newer)
How to remove your iPhone, iPad, or iPod touch passcode without a computer
How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support
Additional:
Optimize the range of the mobile network, see -> 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
Low Power Mode is On.
Charge the iPhone & wait until the battery is fully charged (100%).
The Low Power Mode will turned off automatically.