You can see this video, not English but I think you can understand full problem Video

Yes, Deco sets default WiFi SSID Deco_25:00 after reset When Deco is already to setup (Blink blue led), app cannot find and apply first setup. Your link is broken.

After reset button click, my Deco X50 doesn't full reset. It create Wifi network name Deco_25:00 and I can't setup as main deco or additional deco by Deco App. Deco blinks blue led. Deco X50 Wifi