Thanks for posting, and I would be happy to help with your docking station!
Based on your description I think I understand what is happening. Since the laptop is duplicating your internal laptop display, it is only showing one ‘block’ representing both displays within the Display Settings application (most likely represented as a ‘1’ and a ‘2’ with a vertical bar between the numbers). As a result you cannot make changes to the external display.
Assuming my understanding is correct, we just need to make a quick change within Display Settings to ‘Extend’ your desktop to the monitor connected to the dock so they are both recognized individually (as opposed to being a ‘Duplicate’).
To do so, may I ask you to double-check within the Display Settings application (available via right-clicking on an empty space of your desktop), under the ‘Multiple displays’ section, that the option selected is ‘Extend these displays’? I have attached a screenshot from my system that shows setting in question.
Please let me know if that helps!