I have seen some very old posts about issues with MBP resolution. I am staggered that Dell only have Windows support.  There should be a pre-purchase warning. With MBP touchbar (late 2016) I get 1692 x 3008 both with direct usb-c to displayport, and Apple multiport adapter to hdmi.  HDMI gives a far better image. Monitor shows 'Resolution: 2160p' with hdmi.