Resolution 2560x1440 not available with new DisplayPort to HDMI adapter

I had almost the exact same problem. The issue involve HDMI and DisplayPort cables. The physical resolution of my monitor is 1920x1080. Recently my Apple Thunderbolt 2560x1440 died on me. So, the only monitor I had at hand was an LG with 1920x1080 with both HDMI and DisplayPort ports. I wanted 2506x1440 because I got used to it.

I connected the monitor with an HDMI cable to my Ubuntu 18.04 machine, and all that Ubuntu could see was a maximum of 1920x1080. I tried all the settings with xrandr and still the monitor didn't want to switch. So, instead of HDMI, I got the DisplayPort cable and connected it to the monitor. Ubuntu still sees the maximum physical resolution 1920x1080 but xrandr can set at much higher resolutions. For example, this setting put the monitor at 2560x1440:

xrandr --output DP-2 --auto --scale-from 2560x1440

This is the easiest and faster way to resize logically the resolution of the monitor. More elaborated scripts require to play a little bit more with xrandr,. Like this one:

xrandr --newmode "2464x1383_60.00"  287.50  2464 2640 2904 3344  1383 1386 1396 1434 -hsync +vsync
xrandr --addmode DP-2 2464x1383_60.00
xrandr --output  DP-2 --mode 2464x1383_60.00

You get the long line of settings for --newmode from running cvt. In my case cvt 2458 1383 60.

Although all of this works well -almost well I should say- the only minor nuisance is the quality of the pixelation. That is understandable because my monitor is physically limited at 1920x1080. But that shouldn't be an issue with monitors with real physical resolution of 2560x1440 as your Samsung 27" CH711 is.

Again, the easiest way to see this would work for you is first starting with auto scale, and then move to a more elaborated settings.