Nighthawk M1 intermittently disconnects from the internet

I have 2 Nighthawk M1 Hotspot Routers, both using ATT SIM cards for data.  Both are plugged in without batteries installed.  One has worked perfectly for over a year, the other (recently purchased) will not stay connected to the internet.  It turns off in the middle of movies on Netflix, etc.  I have to physically go hold the button down to turn the wifi back on.  What gives?  

I saw another post with the same problem, but I couldn't understand the solution.  It said something about 'Wifi UI'.  What is a UI? I'm not techie, so please explain in lay terms and if a website is needed, please show which website to access.  Thank you for any assistance as this is an annoying problem for a relatively expensive item.