This is a recurent problem with the gear VR controller, it disconnect when not used for 5min (ie watching a movie) and won't reconnect. There are tons of comments on the web about this problem, like this: https://www.youtube.com/watch?v=ZAVSr88ohgQ
Now it got worse: The controller disconnect even when in use and won't reconnect, or it says it's connected and it's not, do something please, I'm not the only one in that case and it happens after changing the batteries for the 1st time.
I have a galaxy note 9 with the last gear VR version
And please do not ask me if I changed the batteries or if I tried to unpair/pair the controller, OF COURSE I did it.