@MacMartino the deco app should show both wired and wireless devices connected to the network. Any chance he's connected the Xbox to the guest network, or perhaps even a hotspot to get around the parental controls? Can you take a look at the Xbox's network settings to confirm it's definitely connected to your main Deco network?

  @JuzDU i will try. It cannot be a hotspot, since there's no activity on any other devices in the network. I'll try to login to the xbox and check its settings. It used to display the xbox, but since a week it has no activity anymore (and no new device or so). Weird. I'll get back to it when i can login to the settings