@CDH70 Even connected to Deco 1 by a direct Ethernet cable, Deco 2 shows as connected by wifi to Deco 1?  What if you connect a computer by cable to Deco 1?

  @jzchen My provider is Brightspeed (CenturyLink/Quantum Fiber). The fiber comes into the house, plugs into a Calix GigaPoint 803G ONT, which plugs into my C4000XG modem. The modem has 4 ethernet out ports (yellow), of which 3 are feeding my Decos and 1 is feeding my switch.

  @yves_b "Even connected to Deco 1 by a direct Ethernet cable, Deco 2 shows as connected by wifi to Deco 1?  What if you connect a computer by cable to Deco 1?"

That is correct on Deco 1>Deco 2 direct connection. It still shows as connected via WiFi. I have not tried connecting my PC directly to Deco 1, but I will try that this weekend and report back.

Another interesting thing, on my modem where I have all 3 of my Deco's connected, only the port that is feeding Deco 1 has the light on. The other 2 ports where Deco 2 and 3 are connected are not lit up.

I am considering deleting my wireless network entirely from the Deco App and starting from scratch to see if I get a different result. That is a weekend project however, as I have to work during the day and can't be without my wireless access.

  @CDH70 It behaves as if the Ethernet ports of Deco 2 and Deco 3 are defective.

  @jzchen 

Your statement is not correct.

With the Deco in Access Point mode, it is valid to connect all Deco appliances to the modem, and the Deco mesh will use Ethernet as backhaul.

With the Deco is Router mode, it is not valid to connect all Deco appliances to the modem.  In that mode, the main Deco must see the Internet on one Ethernet port, and must not see any other Deco on that same Ethernet port.

  @CDH70 Did you buy the "modem" or did it come with the service?  All three connected to the same node means all three would be set as Access Points, not Mesh.  The instructions note not to connect them this way as you'll create a loop and that is why the Ethernet ports are automatically disabled.

If you purchased the modem it is not needed, connect the Deco directly to the ONT, then the switch also to the main Deco.  Continue to connect the rest of the house to the switch.  Finally connect the 2 remaining Decos to the "sockets".

  @jzchen 

When the Deco is configured in Access Point mode, you can wire satellite Deco nodes to any Deco unit, or the main modem/router.

This is clearly stated in https://www.tp-link.com/us/support/faq/1794/

  @jzchen Where did you read those "instructions"?

It is perhaps an option to eliminate the modem and connect the main Deco to the ONT.  That assumes that the ISP documents how to configure another "modem", and that the Deco is compatible with that configuration. 

We do not know if this is the best option for this case.