I have 3 x20s. One is used as the modem and feeds the lan thru a switch. I connected the other two to the lan and I have no issues with those. I have another lan port that currently is connected to my old wifi router. My wife has an old laptop that cannot connect to the mesh network and this is the only device that uses the old wifi.   I also have an m9 that is not being used. 
 

Can I connect that lan port to a switch and connect the m9 and backhaul thru the lan and also connect the old router to the switch?  I tried connecting the m9 to the port and then connecting the old router to the m9. As I remember, this had some issues.  Or do I have to configure the old router differently?

Thanks