Diagnosis
The most common cause is the router not fully completing its boot cycle before you start the app setup, or the app trying to use cellular data instead of your new Wi-Fi.
How to Fix It
Power cycle everything: unplug modem, unplug router. Wait 30 sec. Plug modem in, wait for ALL lights to be stable. Then plug router in and WAIT until the ring is solid white (can take 3-4 minutes). Only then open the app, ensure your phone is on the new Orbi network, and start setup.
If That Didn't Work
The router may have defaulted to AP mode. Hard reset it (paperclip in reset hole for 7 seconds until magenta flash). After reboot, connect to its default Wi-Fi (on the label) and try setup again.
Heads Up
During initial setup, you must use a phone connected to the Orbi's own Wi-Fi network. The app will fail if your phone is on cellular data or another Wi-Fi network.