Stuck at Signing in after iOS 26 upgrade

I upgraded an iPhone 13 Pro Max from iOS 18.6.2 to iOS 26.0. I got to the welcome screen, entered my passcode, tapped Only Download Automatically, got the Apple Account Security screen, tapped Other Options, and tapped Don't Upgrade. (I don't want 2 factor authenication for now.) Next I got a screen showing (my name) Signing in... with a wait animation. After 10 minutes I powered off the iPhone, powered it back on, got the welcome screen again, and followed the same sequence to the same hung Signing in... screen.

I connected the iPhone to a Mac. Finder shows 23.05 GB Available, Software: iOS 26.0. I clicked Check for Update and got "This version of the iPhone software (26.0) is the current version."

Internet and WiFi are fine. The 5.56 GB iOS 26 update downloaded in 3 minutes. Other devices can use WiFi without problems.

I can log into my Apple account via the Mac App Store and the web. I can log into iCloud via the web.