I read JO's writeup and the comments too. Not sure I feel reassured or not.

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Hi AsusMy

keep getting message 

Reason for failed connection

The network cable is unplugged.
We suggest you

Please ensure that the cable is properly connected between BRT-AC828 WAN port and the modem.
If a problem still persists, manually change the connection settings.

Asus AC2600 BRT828 . ppoe connection to unifi failed, In network map Internet status is network cable is unplugged. WAN led is red.Replace cable from modem to router still got same message. My standard unifi router work well. I have experience configure unifi connection with Asus RT-AC68U, Just key in unifi userid and password to get internet connection. The same way cannot work with BRT828. Is there any other configuration?
Your cooperation is highly appreciated.
Thank you

bought it from china ?

I currently have an AX-3000 (main) paired to a AC-68U (mesh node).

Am thinking of getting an AX-56U to serve as a mesh node on the ground floor.

Is there a reason why the AX-56U is a quad-core router vs. the AX-3000 tri-core??? Is it superior to the AX-3000?