Hello
RB960 HEX POE, with 4x POE (4x UBNT Prism AP) and 1 SFP on it.
57 °C
is that normal?
how can I keep it cool?
I have one of these that sits idle all day doing very little and is still showing 50C - i believe the reported temperature is the silicon die temp, not the actual chip. Don’t worry about it.
My experience is that if you put such case into vertical position instead of keep it laying on the shelf, it reduces the temperature effectively by 8-10 degrees. Try it.
I had the idea to open the case, leaving the board horizontal with the open “roof” 
It may be the second stage if the turning doesn’t help as much as expected.
I have installed another RB960 with 3x POE on it.
I have opened the box and left horizontal with the top open.
still 57° C
the unit is running fine till now.