So OpenGarage is happily connected to Router A (with IP acknowledged by both router and Device)
But any device connected to Router B and Router C can’t reach the IP? But if you turn off B and C forcing everything to talk to Router A (So everyone is on the same device) access works?
This sounds like you have an issue routing requests between routers (which you work around by forcing your clients to the same router by either disabling the others or changing the SSID so you can force a specific router), does access to other devices work cross routers?
If you change the router closest to the device to a new SSID, say ‘Garage’ (leave the others on and alone) and connect the OG device to that what happens with the following
Connect your PC to Garage SSID – does connectivity work (Assuming yes)
Connecting your PC to SSID used by other routers – does connectivity work (assuming this won’t)