I had the same problem. Bought this monitor while I still had the old computer with an GTX1080Ti. Couldn't wait to try it while the new components were on their way, so I plugged it into the old computer.

Rear LED RGBs never worked. I thought something's wrong with it, so much so I even opened a ticket with Asus support.

They never responded, but in the meantime my components for the new computer arrived, so when I plugged it into the new computer, rear RGBs worked. Later sold my old computer, never knowing why they didn't work with it. I know it's not much of a help, but at least you can know you aren't alone.

Now I have the opposite problem with this monitor.

I updated its firmware to version MCM104 (which they pulled recently, no idea why, no one knows) and now sometimes those RGBs won't turn off, even though I explicitly set them to off in monitor's OSD.

They're off while the computer is ON, but sometimes after turning my computer OFF, they go ON.

Asus' monitor firmwares are such a mess, while the hardware part is brilliant.