Unable to pair my Apple Watch SE with iPhone 11

New Apple Watches come with the newest watchOS installed, in this case watchOS 11, which then requires an iPhone with iOS 18 installed.

Apple Watch and iPhone compatibility - Apple Support

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To set up and use a new Apple Watch, you need iPhone XS or later with iOS 18. Learn how to identify your iPhone model and how to update your iPhone to the latest version of iOS.

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copied from: Set up your Apple Watch - Apple Support

Update the iPhone from the Software Update function in Settings or from a computer; pairing any watch on 11 or newer requires 18.

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You can update your iPhone 11 to iOS 18.1, it will not support Apple Intelligence:

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copied from: How to get Apple Intelligence - Apple Support

Can you visit an official Apple Store next to your place? Some users reported, that the Apple Genius Bar people can downgrade watchOS on a new watch, to match your older iOS version on your iPhone.

Apple - Find Locations

Calling the official Apple Support might also work:

Contact Apple for support and service - Apple Support

I have an Apple Watch SE which is perfectly paired with an iPhone 11 running iOS 17.4.1 now and was previously perfectly paired with versions 16 of iOS.