Diagnosis
The node itself is powered on and running, but it's lost its management connection to the main router. This is a communication hiccup, often after a firmware update or internet outage.
How to Fix It
Don't factory reset yet. First, just restart the entire system from the app. In the Asus Router app, go to the main screen, tap the menu (three lines), select Settings -> Administration -> System -> Reboot. This gracefully restarts all nodes. Wait a full 5 minutes for them to come back online and re-sync. The app status should correct itself.
If That Didn't Work
The app's data might be cached wrong. Force close the Asus Router app on your phone and reopen it. If it's still wrong, log into the web interface (router.asus.com) on a computer connected to the network. The status there is often more accurate than the app.
Heads Up
If the node is truly offline, its light would be red or off. A solid white light almost always means it's functioning locally, so your devices connected to that node probably still have internet, even if the app is confused.