ultrawide monitor 21:9 lack of resolution 2560x1080

You may create the new mode resolution using xrandr and gtf.

To get the mode information :

gtf 2560 1080 60 

You might get something like :

# 2560x1080 @ 60.00 Hz (GTF) hsync: 67.08 kHz; pclk: 230.76 MHz
  Modeline "2560x1080_60.00"  230.76  2560 2728 3000 3440  1080 1081 1084 1118  -HSync +Vsync

Then create the new mode using xrandr with the above data :

 xrandr --newmode "2560x1080_60" 230.76  2560 2728 3000 3440  1080 1081 1084 1118  -HSync +Vsync

To add the mode to HDMI-0:

xrandr --addmode HDMI-0 2560x1080_60

Finally to apply the new mode :

xrandr --output HDMI-0 --mode 2560x1080_60

source

Just building up on @harrymc 's answer:

Check your HDMI version (before using gtf) by comparing the release date of your board and those of the HDMI versions, just to discard the possibility of having an old version which does not support such resolution with those refresh rates.

In my case, probably 1.4b, I lost a lot of time messing with outdated drivers and different combinations of modelines, just to get it working with 30Hz (best I could get with my old video card)