After upgrading to Mac OS X "Sierra" my 2K monitor didn't work properly, the highest resolution Mac OS X would drive it at was 1080p. I looked at a number of software fixes, none of them worked. No patches from Apple arrived to fix the issue either.
I read that using a different cable might work. I was using a special HDMI cable and I know DisplayPort has higher resolution on my monitor, so I bought a low-cost Thunderbolt to DisplayPort convertor from Amazon. It works just fine now.