Dell Thunderbolt Dock WD19TB dual monitor troubleshooting

I’ve been using a Dell WD19TB dock at home for over a year now, and it has become one of my favorite pieces of WFH hardware. I can connect all of my peripherals through just a single connection, along with full power delivery to my laptop, a Dell Latitude with a 10th gen i7. Recently, I purchased a 32” LG 4K monitor to act as my primary display, moving my 29” Ultrawide to the side as a secondary. I knew that modern Intel integrated graphics, and the dock itself, could easily support this configuration. What I did not account for is the correct combination of ports and cables you need to get this setup working.

I first plugged in the 4K display over the included DisplayPort 1.2 cable, and the Ultrawide over HDMI 1.4. The 4K display worked just fine, but the Ultrawide was getting capped at 1920×1080, whereas the native resolution is 2560×1080. This was not a problem with the cable, as I had used the same to connect to the dock before I added the 4K display. I then plugged the HDMI into the laptop directly, and the native resolution was immediately available. After sifting through Dell’s documentation, I found out that there is a bandwidth and resolution cap I was hitting by using this combination of HDMI and DisplayPort. To get around this, I needed one source over HDMI, and the other over USB-C. I connected the 4K display to the HDMI 2.0 port on the dock, and the Ultrawide over a HDMI to USB-C cable. At first I plugged it into the front USB Type-C port, as the documentation stated you can’t use the one on the back next to the HDMI port while HDMI is active. The display was unrecognized, and I got a “Unknown USB device warning” in Windows. I then used the Thunderbolt 3 port on the back of the dock (next to the power connector), and the second display immediately appeared.