Deco X20 disconnected and removed from the network, but could not be reconnected back in
I have got 4 Deco X20 in the network in total. One of them at upstairs showed red light and disconnected. I tried refresh; unplug and plug back in then refresh; tried reset button in the bottom. Nothing was working. I remove the unit from the new work, but having problem reconnecting that particular unit. I tried everything methods in the help function. All of suddenly (not sure why and how), the light turn green again., but I can't see this unit in my network.
My questions are
1. Is the internet actually working through that unit? How do I find out?
2. How can I get that unit back onto my current network? It is not showing up on my current network.