2560x1440 for the Dell U2713H over HDMI in Linux

When using the HDMI input on this display, the maximum resolution is 1920x1080, although the panel supports 2560x1440 with 60Hz through DisplayPort.

It can, however, support the same resolution with a 30Hz refresh rate over HDMI. Add the following to /etc/X11/xorg.conf, assuming your external screen is HDMI-2 which you can check with xrandr

Section "Monitor"
 Identifier "HDMI-2"
 DisplaySize 2560 1440 
 Option "DPMS" "true"
 Modeline "2560x1440_30.00" 146.25 2560 2680 2944 3328 1440 1443 1448 1468 -hsync +vsync 

After rebooting the system or restarting the X.Org server the higher resolution should work.