I've been through my Deco program on my iPad, but I can't find how to determin the hardware and firmware versions of my units.  If that's necessary, I'll be glad to add it once I know how.

I have a bit of an unusual setup.  I have a house with 2 Deco M9 Plus devices that is working fine.  (SSID: Hogwarts)  Then I have a 500' fiber optic cable that goes through an underground conduit to our barn (a guest house over 300' away - farther for the conduit, since I had to avoid things).  The barn SSID is Hogsmeade.  I don't think the house LAN will matter, but I'm including it just in case.  I have one DHCP server that's part of my firewall and use the 172.16.01x.xxx address space, so everything on the LAN is in that address space.  The devices in the barn look like they're in the house.  Other than the distance between the buildings, it all acts as one LAN.

Of course the exception is that I need two wifi networks, one for each building.

The problem is the barn is an old cinderblock building that has been renovated.  (And, thankfully, all the pig and pig poop smells and leftovers from 20 years ago have been removed!)  I have almost a typical three unit M9 setup there.  I have a main one, close to the center of the building, one M9 on one end, near a 2nd floor window, that is working fine, and a third M9, also on the 2nd floor, that is usually disconnected.  All three are connected by ethernet cabling, through a 16 port TP-Link switch.

Due to the nature of this building (the cinder block walls), reception can sometimes be iffy.  To get thorough buiding coverage, I need to either put this M9 where it is or buy another and use it as a link.

Isn't there a mode to keep all three M9s connected through the ethernet connection so even when the wifi connection breaks down, this third M9 will provide wifi for this half of the 2nd floor of the building?