Precision M5550 not displaying correct maximum resolution

PC: Precision M5550
Three monitors: Dell P2721Q 4K (two), Dell P2222H (one)
Cables: DP to DP for the 4K monitors, and HDMI to HDMI for the smaller monitor
Docking station: Dell Thunderbolt Dock WD19TBS
Laptop lid is closed during use, so only three monitors active

One of the 4K monitors displays with maximum 3840 × 2160 resolution (expected), but the second 4K monitor only displays up to 2560 x 1440 resolution (not expected!). The third smaller monitor resolution is not important.

We want both of the 4K monitors to display 3840 × 2160 resolution. I believe our hardware is compatible? Are we using the wrong cabling? Thank you!