Nighthawk App - can't pause devices - AX3000 AX4 Wifi 6 router (phone app shows RAX35)

I have this:  NETGEAR Nighthawk AX4 4-Stream AX3000 Wi-Fi 6 Router (RAX35-100NAS)

House is 1929 - concrete-like plaster - 2 story - 1900sf

I'm having issues streaming on my  1st fl TV, which is a floor below the modem and router.  I generally have 10-13 devices running at any given time. I'm assuming I need to disable some devices to free up bandwidth. The sliders on my app aren't working. When I try, it "thinks about it" and then flashes "please try again later".  How can I fix this? 

I'm also wondering if a wireless extender might help? During the day especially, my work computer often drops connection, making it impossible to conduct video calls with others in my office. If so, what would you recommend as far as extenders go?  

(FYI - I am NOT very tech savvy, so please be as specific as possible in your suggestions without a lot of tech jargon because I won't be able to follow it otherwise.)