Help with 14.9 monitor

I have a rx 480 and a 14.9 long monitor.
Window only show 1280x960, 1280x 768 and 1024x768 as resolution, it suppose to be 1280x390.
I tried set 1280x390 at 68hz. I saw someone have luck with that, after run the restart64 or restart. The custom resolution didnt show up. I am not sure what went wrong.
What do I need to do?