Hello @KeithB3636
Google/Nest WiFi use a (hidden) 802.11s mesh WiFi to interconnect. There isn't a single "topology" per se, but there is a path for traffic between any two points. Each step (hop) on that path is decided by the unit that currently has the packet in question. To decide which unit to send the packet to next, it uses a table that maps all of the known final destinations to the "best" next-hop mesh unit. To build this table, it needs some kind of metric – something that can be measured, but also something that doesn't change quickly. The 802.11s mesh implementation uses "hop count" as that metric. Using signal quality wouldn't work, since there's no way to compute the end-to-end signal quality – only the single-hop signal qualities can be directly measured. In short, the algorithm picks the next unit based on which one will get the packet to its destination in the fewest hops. Keep in mind that since there's a single 5GHz channel being used for all of the mesh traffic (and any 5GHz WiFi client traffic), multiple hops would just retransmit the data back out on the same channel again, using more channel time. In a typical home, this "fewest hops" approach works quite well, and that's the environment these were designed to be used in.
With that as background, to your situation, getting coverage to outbuildings via the 5GHz WiFi mesh interconnect is not going to be reliable in most situations. Running a physical Ethernet cable works well, putting inexpensive Ethernet switches on each end of some additional electrical isolation.
Using devices from someone like TP Link that have high gain directional antennas can work as well. You may not even have to mount them on the outside of the buildings if you can arrange to have them placed in windows on each end. I don't know why you'd have to crawl under anything for that, but you know more about your situation than I do. They then look like Ethernet on each end, and you can connect a Google WiFi unit as part of your system out there. This is more expensive in terms of materials than cabling, but may end up cheaper on a total cost basis. Configuring them isn't as easy, though.
So, that's my advice. In a nutshell, don't try to make the mesh network do something it wasn't designed to do. For best results, run an Ethernet cable. If that's not possible, use WiFi bridge products from someone like TP Link to create a virtual Ethernet cable.
Good luck!
