Very simply put:
I have managed to get RSR working ONLY when I disconnect my secondary (and tertiary) monitor. As seen on the first image, only the LG is connected to the PC.
It does NOT work when I connect my secondary monitor, as seen in the second screenshot. I have not been able to find any info online, so I created my own post.
Main gaming display is 27" 1440p. The other display(s) are 1080p 60 hz.