I have been using the m9 plus devices for a few years now. My main deco is connected to my second one with an Ethernet cable ( Backhaul) and the third one is connected to the second one wireless. First one from a location perspective is unreachable to the main deco unit. 
 

I think after the upgrade to latest version the third one does not connect to the second one as it has for years.  It insists on connecting to the main deco. I tried everything from "reset and add new" to selecting preferred signal source as the second deco. Nothing worked.

 Any ideas?