Adjusting brightness and contrast on an ASUS monitor?

I just had this problem and discovered it was because I had "eco" mode turned on (under system setup).

Since this came up as a first page result for my search for "asus tuf gaming vg289 brightness",

I found in my case it was because Mac OS had defaulted "High Dynamic Range" to enabled. If you disable that, then you can configure all the brightness settings with monitor's buttons.

It's been 3.5 years, and I have a different Asus monitor, but to fix this I disabled Game mode and Eco mode in the system setup, and changed the splendid mode to standard. After that I was able to adjust brightness and contrast separately.