Why doesn’t my Apple Watch 3 want to pair up with my iPhone 11pro max ios 16.5?

If you have iOS16 or later on the iphone, series 1 to 3 Watches are difficult to pair because of relatively low storage but…

Ensure the iphone is on iOS16.4 or later.

Erase the Watch completely. Put your watch on its charger and keep it there until you complete these steps. (select the watch OS version) Erase Apple Watch – Apple Support (UK) 

When pairing do not try to restore the backup, pair as a New Watch Set up your Apple Watch – Apple Support (UK)

The FieryJack article is out of date. It was fixed in ios16.3.1

[Edited by Moderator]