Update:

this morning, after a long struggle to figure out why the network is excruciatingly slow, I eventually tracked it to the RE505X units - and in particular, the fact that they are connected to the LAN via cable as well. I had to remove the LAN cable from the units before the network speed returned to normal.

Can someone please help me - is there something specific that needs to be done when connecting/configuring the RE505 to be part of the MESH? In all the marketing material, it states that for extender function, you simply plug the RE505 into the LAN, but it seems this is not compatible if you want to use it as part of the mesh instead?

@Kevin_Z 

Thanks for the clarification - I guess it is what it is - and to be honest, it's disappointing since I'm assuming that each of the RE505x devices must then be within WiFi coverage of the router - or can they daisy chain off each other? The furthest room in my house is not within range at all - so how do I get the MESH to extend further?