I would break things down into smaller steps with tests in between. So, first, make sure that boathouse Nest WiFi Router unit has been added to the wireless mesh in the house. Give it a chance to update its firmware if needed, etc. Then, connect it via Ethernet to the switch (which is, presumably, connected to the LAN Ethernet port on your primary Nest WiFi Router unit). Use the Google Home app to verify that the new unit is connecting "wired" instead of "mesh". Then connect your boathouse powerline adapter to another outlet in the house and connect the new Nest WiFi unit to that instead of directly to the switch. Verify "wired" in the Google Home app once again. Finally, move the powerline adapter and new Nest WiFi unit out to the boathouse. If it has worked at every step up to this point, but doesn't work out there, then focus on the powerline adapter (maybe use a laptop or something to test that). If it fails at any of the earlier steps, you can focus on solving those problems in a simpler environment. I hope this helps!
