Try this:
Create a backup copy of your settings-ce.cfg
Write-protect your backup (be fool proof for your own safety)
1) Open settings-ce.cfg
2) Find the following lines [my settings (4K resolution), insert your resolution values]:
r_Height = 2160
r_HeightWin = 2132
r_Width = 3840
r_WidthWin = 3834
r_Fullscreen = 1
r_WindowedFullscreen = 1
3) Save the file with your new values before closing the settings-ce.cfg file!
4) Now write-protect your settings-ce.cfg file
5) Launch Client Loader and log in
6) Ingame in this order go to
Options > Graphics Advanced > Advanced Resolution Mode [ yes ]
Apply
Options > Graphics > Fullscreen Resolution > Auto
Options > Graphics > Fullscreen > [ yes ]
Apply > OK
It is quite possible, that Step 6) has to be made once as your first step.
Sorry, i dont remember.