Please see the attached photo for my topology.   My Gateway is a Pro6.  All other eeros are the old school original eero mesh units (J010011).  All software is up to date.

All eeros are wirelessly connected.....with the exception of C and G1.  The garage is a detached garage, with a second story apartment and is about 100' from the house, so wifi signal wouldn't reach the garage.  To fix this, I ran approx 150' of Cat6 cable and connected eero C to eero G1, and this has worked great giving the garage wifi speeds between 90-120mbps.   I've recently added Eero G2 as I'm needing wifi signal for some equipment on the side of the garage and G1's signal doesn't quite reach reliably.   G1 is on the second floor in the apartment.  G2 in on first floor in the garage.  Straight line they are around 25-30ft apart.  Mostly drywall/floor joists and carpet between the two.  Nothing crazy.   G2 gets zero bars of wifi......maybe a single one if I'm lucky.  With G2 unplugged and me standing in the same spot, my phone connects to G1 with no problem and gets great signal and speed.  I've moved G2 to various locations on the first floor of the garage....even directly below G1 and same thing every time.

To me, it seems like G2 is actually connecting to B or C and just far enough away to where the signal is unusable.  I've played musical chairs with the eeros and the same thing happens every time.....the issue does not follow a specific eero. 

Is there something with my ethernet backhaul setup that is causing the issue? 

Bonus Quirk:  G1 shows up as "wired" with a  1Gbps wired data rate, however, from time to time it does swap over to "wireless" usually with 1 bar....sometimes zero....sometimes 2....it stays there for a few seconds and then goes back to wired.