@Proflashrtist 

The units are identical, you can use any as main unit.

You can use any Ethernet port for WAN, the Deco will detect if it is used for WAN or for LAN.
 

Turn them all off. Then hold in the reset button and power one on only - the one with internet connected to it. If you want it to be an access point, instead of a router, make you specify that when asks for connection type.

Once you have internet working through it, then power on and add the other decos.

  @Proflashrtist I managed to get the network up and running by just letting the setup each time continue to fail and restarting the app. Unfortunately this network is so unstable I think I am going to return it and go back to my ASUS setup with the AX11000 and a satellite node upstairs. It's a little slower, but I never get wifi drops. Since I set this up a few days ago I have completely lost the home network almost a dozen times. Since I have to travel a lot for work and want to see my cameras this just won't cut it. I think it's going back to costco. 

  @Proflashrtist Did you manage to let it update the firmware? It was notoriously unstable in early builds. It's gotten a lot better, I think the latest build is 1.2.9 for the XE5300 from Costco.