Is there a guide regarding how the ethernet ports of the Deco X55 should be used?

I have two units in AP mode with ethernet backhaul and this has been working very well for over a year.

I recently connected a device to one of the additional ports on one unit, but that seemed to have cause instability. At times the device I plugged in falls off the network. Other times the Deco with this extra device falls off the network; at these times the light stays green, but the unit does not show in the app and Wi-Fi devices that are too far from the first Deco are not accessible.

Are there certain ways in which the ethernet ports should be used?