Thanks for posting. I’d be happy to help with your docking station!
The cable you are using with a HDMI connector on one end and the DisplayPort connector on the other is most likely a passive cable. This cable will only allow the conversion of a DisplayPort output to a HDMI input, not the other way around as you are trying to do.
Does your monitor with the DisplayPort input also have a DVI input? If that is the case you could use a HDMI to DVI cable as those cables work in both directions.
Let us know if that information helps when you have a moment and we’ll go from there.