for me the Steam game is "stuck" at 720p resolution regardless whicht setting i choose(3840*2160) which is very frustrating.....So good thing I haven't uninstalled the Window store version.
only thing i can think on this is going to be your desktop resolution. by default, the game only allows windowed and borderless windowed/virtual desktop. in either case, the game won't go past the resolution set by your desktop.
two things i can suggest...
A.) change your desktop resolution
B.) goto your Documents folder and goto SEGA > PHANTASYSTARONLINE2_NA_STEAM. (or the other one if you are using MSStore version) open user.pso2 in notepad. go all the way to the bottom and you should find some resolution options. if using it, change the virtual option to false and the other fullscreen option to true.
you will find some height and width options, but i don't know what the "3d" options do. they don't seem to do anything. either way, i wouldn't change anything you don't understand without making a backup copy.