Nighthawk AX8 Mesh Extender/keeping wireless device connected to router only

[reedaf4]

My Netgear Mighthawk AX-8 Mesh Extender is taking over some of my wireless devices from my RAX120 Nighthawk Router, one of which is my Chromecast device.  When that happens, my Chromecast device no longer works for casting live TV channels.  I have to reboot the extender and allow the router to pick up the Chromecast device again, and then it works for casting TV channels.  I don't know how to prevent or lock in the Chromecase device to my router, and not allow the extender to pick it up.  My extender is not set up as an access point, and my router has UPnP enabled.  These were the two recommendations I received during Chromecase setup to check when I tried to reprogram Chromecast, and it was not connecting to my Wi-Fi network.  Any help would be welcome.

[plemans]

I don't have the EAX80 but I do have the EX8000. 

It has an access controll setting under settings---->connected devices:---> then click "enable access control". 

Then you can deny devices from the extender. 

If it has that option, try it and see if it works for you.