How do I change the resolution for the external monitor to 2048x1152 HiDPI?
The Dell P2421DC is a 24&quot; 2560 x 1440 display. You can set your display to use 2048 x 1152 HiDPI, which will use the full pixel range, but scale objects up, making them appear larger. Whether text will be clearer will have to be seen. You'll also be taxing your graphics processing by scaling to a 1.25 factor, rather than whole number pixels.
Hold down ALT when you click on the 'Scaling' radio button in System Preferences &gt; Displays to produce more options.
Your monitor has has pixel density of 122 pixels per inch. A Mac Retina display typically has a density of 218 ppi. You're never going to get close to a Retina display clarity with that hardware.
Earlier MacOS versions did have better smoothing for lower-density displays, but this has been removed, for reasons that only Apple knows about.