DIAGNOSIS The new unit connected to the mesh network as a client, not in setup mode. The solid blue light confirms it's already on the network but not adopted.
FIX Factory reset the new unit. Find the reset button on the bottom. Press and hold it with a paperclip for about 10 seconds until the LED flashes amber. Release. The LED will then pulse blue, indicating it's in pairing mode. Now use the Deco app's 'Add a New Deco' function.
IF THAT DIDN'T WORK Your phone may be connected to a different WiFi network during setup. Ensure your phone's WiFi is connected to your existing Deco network before starting the 'Add a New Deco' process in the app.
HEADS UP The Deco XE75 uses WPA3 encryption by default. If you are adding an older, non-XE75 Deco model to the mesh, compatibility issues can occur. The system works best with all identical units.