Biggest resolution on my Mini (silent upgrade) is 1920x1080. There is more possible, but my monitor doesn't allow vertical frequences lower than 56Hz.......
MOS reads monitors frequency range at startup and calculates possible resolutions. If you go to Sysprefs/monitors and click on "new" (for new screen-modi), you can see all resolutions calculated. Some of them may not work on your monitor. You see also the frequency-range of your monitor by clicking on "edit" (ignore safety message). Compare them to your monitors technical data - maybe they where read wrong......