Another possible issue that it just took me 4+h to figure out...
My housemate set up our eero network with identical name and password as our original network. I recommend disambiguating the two (just add `[eero] or something to the name), because on all of my devices (imac, mpb, iphone, android), only one instance was showing.

The result is that when I was configuring Sonos in one room, devices were on one network. But then moving myself and/or speakers to different rooms meant the device(s) were switching between the original or eero networks. But because the network was showing up with the same name, I couldn't tell. I'm far from technical, so don't know if my explanation makes any technical sense. But as soon as I renamed the eero network, I was able to see which my phone was connected to, which my sonos was configured against, and that one of my speakers was connected to a different network.

So my tl;dr:  make sure to name your eero network differently than your original network.

Otherwise your devices and/or sonos system may be switching between them during/after setup without your realizing, resulting in your burning most of a day driving yourself insane trying to understand why the devices show as connected to a network (solid white light), but don't appear in your app