4K monitor connected with Displayport loses signal as soon as laptop screen turns off

Question

I just bought a new 27” 4K screen (BenQ EL2870U) to use in my office. Since there was no DisplayPort cable included in the box, I started connecting the screen via HDMI to my laptop (Lenovo T460p in a docking station; cable connected to docking station). However, since I can only use 4K resolution with 30 frames per second, I decided to switch to DisplayPort. Easier said than done…

Once I connect the screen via DisplayPort to the docking station, I am experiencing some strange behavior. The monitor is detected but not working properly. Everything is fine as long as I keep the laptop open in the docking station and either duplicate or extend the screen. Once I turn off the laptop screen, the monitor loses signal. That does not only happen when I close the lid (I have selected ‘do nothing when closing the lid’ in the options) but also when I select ‘show only on Screen 2’ after pressing WIN + P. This also turns off the laptop monitor but at the same time the external 4K monitor loses the signal.

I have already tried both DP ports at the docking station, changed the screen priority multiple times, and changed the BIOS to use the external monitor as the preferred screen. This actually works during the boot sequence; the laptop can remain closed and everything is displayed correctly on the external monitor. Everything worked perfect while the monitor was connected via HDMI.

My goal: I want to connect my laptop to the docking station while it is turned off, turn on the computer in the docking station and only use the external monitor (with DP).

Any ideas?