I was reading the FAQ and came across this:
I use a WiFi extender, will PlexConnect still work?: Why not? 
I can confirm that Plexconnect works with my wifi extender. I use a Netgear WN3000RP to extend my main home network. The extended network is named the same as the main network but has the “_EXT” suffix.
The ATV I use to test Plexconnect is on the extended network and everything works as it should. I do have my router set to give the ATV a DHCP reservation, not sure if that matters or not.