I just bought two M5 units and I think one of them is defective. 

I tried to setup it up as the main unit and it failed because I could hardly get wifi signal from it (it was super weak). Then I decided to setup the other one as the main and this one as a slave and it only works when the two devices are one meter away (or connected via Ethernet), otherwise it would start a red blinking light. I believe the issue is with the wifi only because I connected my laptop to it with an Ethernet cable and it worked great.

I tried to reset it a few times with the reset button. 

The other unit works really good. 

The main unit is connected directly to the modem.