Hello Luc Truntzler,
First off, we need to clarify something – the Nest WiFi Point units don't have Ethernet ports at all. So, they can only be connected via the wireless mesh. The Nest WiFi Router units have Ethernet ports, and you can connect another Nest WiFi Router unit as a secondary, and if you do that, you can connect it to the primary via Ethernet. When you do that, you should be able to use either port on the secondary, but I would probably connect it via the WAN port and stick with that.
When you connect a secondary to the primary via Ethernet, it will use that path to send all client traffic to and from the primary – the wireless mesh connection will still be there, but won't be used (unless the Ethernet connection fails or is disconnected).
Just a couple other pieces of advice when doing this. First, if you use one or more Ethernet switches between the primary and any secondaries, make sure they are inexpensive unmanaged switches. This is to avoid issues with the loop detection features many managed switches have enabled by default.
Second, when you connect a secondary via Ethernet, it will not act like a "base station" for more distant wireless-only / mesh secondaries.
I hope this helps – please feel free to ask for any more clarification.
-From MichaelP, Gold Product Expert.
