Can I get 2560x1440 resolution on external monitor? I have a mid-2015 MacBook Pro running High Sierra. I normally use it as a desktop computer in clamshell mode with an external monitor.

I just replaced my 24-inch external monitor with a 32-inch monitor. The new monitor is an Acer EB321HQU-B, which is supposed to support up to 2560x1440 resolution. 

According to the technical specifications my Mac is supposed to support


  full native resolution on the built-in display and up to 3840 by 2160 pixels on up to two external displays, both at millions of colors.


and


  Support for 3840-by-2160 resolution at 30Hz


I have the new monitor connected via a Mini DisplayPort to HDMI adapter.

But the best I can get is 1920x1080, which is the same as my old monitor. In Display preferences, the only options I see in Scaled are 1600x900, 1080p, 1080i, and 720p, whether I select 60Hz or 30Hz. So I can't show any more on the screen, it's just all scaled up 33%.

Is there some way I can take advantage of the new monitor?
The problem is the cable you’re using to connect the monitor to the computer.

The easiest way to ensure that you have the full resolution is to use a standard mini-DisplayPort to DisplayPort cable and you should be able to use the full resolution without problems.

You can also use a mini-DisplayPort to DVI cable, but these are usually harder to come by in a standard home electronics store.

Last but not least, it might be possible to get the full resolution by using a better HDMI cable. You'll need one that supports HDMI 1.3 or 1.4 to support the 2560x1440 resolution.