iCloud Tabs is a wonderfully useful feature in Safari for iOS and MacOS. It allows us to view what tabs we have open all our other devices and close them if necessary (swipe left). Unfortunately, after I restored my iPhone X iTunes backup to the iPhone XS, the new phone would not show up in iCloud Tabs. The fix is really simple…

1. Make sure you have Safari syncing enabled under Settings → [Your Name] → iCloud.
2. Close all tabs on the device which is not showing up (XS in my case).
3. Force quit Safari on that device by opening the App Switcher (swipe up and hold from the bottom of the screen) and then swipe up on Safari’s card to kill it (if you’re still on iOS 11, you will need to hold the Safari card until a red ‘x’ appears, then click on it).
4. Open a new webpage in Safari on affected device.
5. Check iCloud Tabs on another device.

Your Safari iCloud Tabs should start showing a few seconds later.

If not, then you can always try disabling and enabling Safari sync under iCloud’s settings or logging out and back in to iCloud (which I recommend as a last resort). Some people also recommend force restarting your iPhone or iPad — I don’t know if this helps or not.