@Greg_G 

>Is this expected behavior in such configuration,

No, it is not. Your configuration should work. I have Main and Satellite Deco wired to ISP router, they work just fine, no reports of disconnection.

Usual suspects are Deco node configuration, wiring issues, router configuration.

You can start with simple test: power off both Satellite Decos, disconnect one of them from Ethernet cable and bring it to where Main Deco is, power it on without connecting to Ethernet. After Satellite Deco starts it will see Main Deco and should try to connect to it by WiFi. Wait till light goes green, give Deco app a minute or two to get updated status, and check if your Satellite Deco is reported as healthy now.

That would eliminate Ethernet wiring as suspect.

@Alexandre. 

Thank you for your reply
 

Well, from what I've tried, there is no issue over WiFi. When satellites are disconnected from Ethernet, after a few seconds they connect to the main one, and properly register in app as online. I suppose I could leave them wireless, but given the building's construction, that's less than ideal... I really wanted to take advantage of existing cables and wired backhaul.

But I also noticed something interesting. When I plug the Ethernet cable again, satellite's LED changes to red for a few seconds, then to solid green.

In the Deco app during this time I can see that:

- satellite's signal source updates from "Main Deco,  2,4GHz / 5GHz" to "Main Deco, Ethernet", which suggests that Ethernet Backhaul is on,

- clients connected to the satellite, both wired and wireless are properly listed

Sadly, after another ten or so seconds, but app again starts to report "Disconnected from other Deco units", and stays like that. LED stays green. Client devices don't seem to mind at all, and they keep the same IP addresses whether their Deco is on wireless or wired connection.

At first I was hoping that maybe it's only the app that reports wrong status... but then again, web UI available at http://tplinkdeco.net shows exactly the same thing. So perhaps ISP router configuration somehow blocks Deco's from recognizing each other in "star" topology... Any idea as to what settings could affect this?

@Greg_G 

> perhaps ISP router configuration somehow blocks Deco's from recognizing each other in "star" topology

You can test that theory. Bring one of your Satellite Deco to ISP router and connect to router using short Ethernet cable of good quality. The best would be to use cable that came with Decos. If you use it for Main Deco, swap it for Main Deco with another cable and take Deco's cable to connect Satellite Deco to router.

If the problem is with the router configuration, even in that setup you'll have issue with Satellite Deco. If Satellite Deco suddenly looks good in all aspects, while wired to router with short Ethernet cable, it must be wiring.

As for settings on ISP router, do you by any chance have MAC address filtering (blacklisting or whitelisting), parental controls on, list of URLs to connect/not to connect, DNS server different from default? These would be first suspects.