I've a samsung s9+ with original Dex station (not dex pad).
In resolution chooser i've only automatic, FHD and HD+ selectable. WQHD (2560x1440) is in gray and non selectable. I've tried both my monitor 2k and tv 4k. Max is 1080p.
In one s8 tread someone with the oreo update can switch 2k with this station.