Thanks for posting. I’d be happy to help with your docking station!
Given your description, the first thing to check is if Windows is just extending your desktop to the second display with a black background with no icons or taskbar. If you try to move windows or applications between the connected displays does the HDMI monitor work as expected?
If that helps, great! If not the next step is to double-check that the correct source input on the HDMI monitor is selected. If that checks out, the next step is to rule out a faulty or incompatible cable. Can you confirm you are using a standard HDMI cable with a HDMI connector on both ends? Would you happen to have another HDMI cable to swap in and test with?
Let us know if that information helps when you have a moment and we’ll go from there.