@Dave_M_ 

The answer is yes, you can hardwire the units together. That's called a wired or ethernet backhaul and is the best way to set these up.

Are you using them in access point mode with an existing router (not the modem) or do you have one M5 set up as the main router connected to your modem and the other units as satellites?

If you are using the M5 in router mode, then you would need to connect the satellites to a network switch that then connects to the main M5 (a star configuration) or you would daisy chain the M5s together, depending on the wiring you have available.

I have 3 M9 units in my house, all connected over ethernet via network switches and it works very well. Great WiFi coverage and speed everywhere in the house.

@Goose91 

I have the same issue.  If I turn them off and on or swap the ports I can get any mix of ethernet and wifi backhaul but not all ethernet.