Hello @Erasmian
The Google WiFi unit on your ground floor is the primary. It is connected to the "outer" network created by your incoming router, and then it creates a new "inner" network for your WiFi clients. But, that "inner" network is also available through the "LAN" Ethernet port on that primary Google WiFi unit.
With all of that as context, you can connect a remote/secondary Google WiFi unit via Ethernet, but it has to be connected to the "inner" network – not to the same "outer" network that the primary Google WiFi unit is connected to.
So, if you can arrange to have that Ethernet cable connect to the LAN port on the primary Google WiFi unit instead of to the "outer" network, then you can connect one or more of those remote secondary Google WiFi units to that switch to use Ethernet as the interconnect, which should improve performance significantly.
But, it is important to note that when you do this, all of the other wired devices connected to that switch will now be connected to the inner network as well. That isn't normally a problem – they may need to be rebooted, for example. But, they will be on a different network than they were before, so they may no longer have easy access to anything else still connected to that outer network. However, in general, it's better to have everything (wired and wireless) be on the same network as each other, so it may be an improvement.
