HDMI-connected monitors natively support only resolutions up to 1920×1080, even if they are capable of 2560×1080.
There are multiple solutions available: check this page.
- @ 55 Hz with reduced blanking:
xrandr --newmode "2560x1440" 220.812 2560 2608 2640 2720 1440 1443 1448 1478 -hsync -vsync
xrandr --addmode HDMI2 "2560x1440"
xrandr --output HDMI1 --mode 2560x1440 - @ 30 Hz:
cvt 2560 1440 30
xrandr --newmode "2560x1440_30.00" 146.25 2560 2680 2944 3328 1440 1443 1448 1468 -hsync +vsync
xrandr --addmode HDMI2 "2560x1440_30.00"
xrandr --output HDMI1 --mode 2560x1440_30.00