@David-TP the main Deco XE5300 is wired on one end to the Comcast modem and on the other wired to the NetGear GS108 in the same utility closet. The other 7 ports of that NetGear GS108 are connected to the home Ethernet wires for 7 different rooms. Two of those rooms outlets have the AP Deco XE5300. One of those rooms is my home office, and the outlet is connected to my second NetGear GS108.

Because all the Deco are connected to the main NetGear GS108 switch, either directly inside the utility closet for the main Deco XE5300, or via the house' Ethernet wiring, I expect the Deco to communicate via this Ethernet backbone and not through WiFi. Hence the linked article Manual Signal Source Selection on Deco - Home Network Community (tp-link.com) did not appear relevant to my issue.

I have already attempted to unplug and reconnect the main Deco to the main NetGear GS108 switch in the utility closet, as well as cut the power to all the equipment - modem, router, switch -, let it cool off for a minute then restart it from modem, then router, then switch. No success restoring the Ethernet connectivity.

I need the main NetGear GS108 switch in the utility closet behind the router and before the house' hard wiring, by design as there needs to be a switch or a router with 8 ethernet ports to connect the whole home. The satellite XE5300 are in different rooms where it is meaningful for them to extend the WiFi coverage in AP mode. It does not make sense for me to have Ethernet switches in these other areas where the satellites are connected.

In the Android Deco app, I see a blue cable logo atop the connections between the main Deco and the satellite Deco. Does that mean that the Decos are indeed using the Ethernet backbone between themselves, just that all the other Ethernet devices on the backbone are getting denied DHCP service from the router Deco?