I think this should help you and all rasbperry pi users (no modification to any text files is necessary), I ran into the same problem, where the default was 640x480 resolution, the monitor was unknown and I ended up using xrandr at first and all the other stuff which didn't work. But I realized that ubuntu mate like raspbian does have raspi-config available so you can do is go into the mate terminal then type
then you can choose advanced settings then Resolution, you should be able to select optimal resolution settings. After rebooting the changes should take affect. Well for me I chose the 1920x1080 and oddly enough when I checked the display settings it wasn't monitor: unknown anymore but my samsung monitor this time was recognized, you can view it here, which allowed me to choose from lots of resolutions so I hope this helps the readers too.