Knocked off Google Chrome Casting

BACK GROUND:
High School classrooom on college campus. (Career and Tech)

GEAR:
router- Nighthawk X10 AD7200 Smart Wifi Router that gains network access from college network. The router is only a year old. We use this to supply our own wifi and internet isolating us from the college students.
We also have the Nighthawk X6S extender EX8000 to extend into a second classroom.

computers- we use Google ChromeBoxes on a Google Education Suite Domain for logins.
Students do have the ability to Cast, BUT don't. I'm not even sure any of them know how.

Google Chromecast- BRAND NEW! the latest chromecast puck 4k version; we have two of them connect to two seperate 55 inch tv's. HDMI (seems like the second one in the back room does'nt disconnect)

 

MAIN PROBLEM:  Recently, I am getting knocked of the wifi connection to the Chromecast. It will play for several minutes and then will loose access, and sometimes internet access. In the LOG file I'm seeing a MAC address that cannot gain an IP because I have Access Control turned on. I cant find this device anywhere physically. Timing makes me think it is it.
I've got the barebones running on the router. NO Beamforming (once thought this was the problem) and no other fancy routing. Just DHCP to a few devices. It also seems like its when the router logs itself out. (Does anyone know how to set the router to stay logged in?)

I've put in a work order to the college but have not heard back from them.

 

What is very peculiar is that I have ran chrome cast for years, and only this year has it began doing this loss of connection thing. At one point I thought it may have also been the fact that my phone was randomly using a different MAC address when it logged in of a morning. I turned that off and set it to use the phones MAC. Did'nt help. I'm fairly sure its not a student playing.

 

ANYBODY HAVE ANY IDEAS????