DIAGNOSIS The new unit is not on the same firmware version or network segment as your existing mesh. It cannot communicate.
FIX Factory reset the new Deco. Hold the reset button for 10 seconds until the LED flashes amber. Release. Now, with your phone connected to your existing Deco WiFi, open the app. Tap Add a New Device. The app will guide you. Place the new unit in the same room as the main unit for setup.
IF THAT DIDN'T WORK Your phone may be switching networks. Disable cellular data during setup. Ensure Bluetooth is enabled on your phone, as the app uses it for initial discovery. Restart your phone's Bluetooth if the app stalls.
HEADS UP The Deco X55 system supports up to 10 units. Adding too many in a small space can cause interference and reduce performance. Place new units to fill coverage gaps, not directly next to existing ones.