Apple Watch Series 9 will not sync activity or data

The checks in more detail

Health and Activity data needs iCloud Health on if you want it on other devices, and Fitness data sends to the iPhone when the connection returns. Apps that show on the watch must be installed on the iPhone, and Settings in the Watch app controls which appear. Notifications reach the watch only when the iPhone is locked and the watch is on your wrist and unlocked. Large backlogs take minutes to appear. If the watch will not reconnect, unpair and pair again with a backup. A Series 9 that never syncs after a restart should be looked at by Apple Support.

Check the watch's Settings, General, Background App Refresh and the iPhone's Watch app settings for which apps show on the watch; an app that is installed on the iPhone but not on the watch will not sync until you add it. If Activity rings are not updating, restart both devices and open the Fitness app on the iPhone, which wakes the sync. If Health data is missing across devices, check that iCloud Health is on in Settings, your name, iCloud. Be careful about restoring or changing the iPhone's Apple Account, because a watch is tied to the account and a changed account breaks the link;

sign in with the same account on both. If you use a work iPhone with restrictions, an administrator may block syncing of Health data. Check Settings, Privacy and Security, Health, for the watch's data sources and permissions, which controls what the watch can write to the Health app. If syncing stops after you change the iPhone's name or Wi-Fi network, restart both devices, and check that the iPhone is not in Low Power Mode, which pauses some background syncing. Plug in the iPhone and keep the watch near, then open the Watch app and the Health app to prompt a refresh.