It seems like Intel Custom Resolution only allows you to make resolution lower than your current maximum resolution, such as 800x450 or 1024x576.
My Display is 1366x768 and when I tried making 1600x900 it doesn't work (even used other app for that), so yeah, no downscaling from higher resolution.
Well it seems that i need to talk to the computer manufacturer to see if i can add the custom modes feature or not. So today ill try to get a hold of them and post back if i come to any solution in case it might help anyone else.
Yeah, you're right about that. The only fix, since the limitation is put in place because of my graphics card being the on board one, is to buy a replacement graphics card and install the drivers for that.