Firstly check that xrandr has the modeline you need (resolution of the screen you want to use)
$ xrandr
Screen 0: minimum 320 x 200, current 2732 x 768, maximum 16384 x 16384
eDP connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 173mm
1366x768 60.00*+
1280x720 59.86