On one floor of my house, one of the Deco units is plugged into a GigE switch and a MoCA box is plugged into the same switch.  It works fine -- the router sees the Deco unit connected via Ethernet.  If I plug a laptop into the switch, it gets on the network quickly and the WiFi works as well.  So far so good.

I have a Synology NAS in the same room that I want to connect as well.  But if I connect it to the switch, the Deco never sees it.  If I plug the NAS into the Deco's other port directly, it come right up.  I've tried different switch ports and cables, but it makes no difference.  Before the Deco, I had an Orbi system that worked perfectly with the switch and the NAS.

The reason I want it this way is so that if I connect my laptop to the switch, it'll be able to access the NAS woithout having to go through the Deco.