Do #3 here. Then retest.
  OK, I am baffled. I have a Lenovo P51S Laptop, Ultradock 3 90W and a Dell UP2516D monitor. I have the monitor connected via mDP to the dock and it works as expected EXCEPT for a known issue with no solution where the monitor will blank and then lose signal when using the DEVELOP module in Lighroom. I doubt you can fix that, so I attempted to as it seems it only happens with a DP connection. I bought a 4K HDMI cable. Connected to dock and monitor. Selected HDMI on monitor and no signal detected. The tried another known working HDMI cable and no signal detected. Tried both ports on the monitor. Then tried direct from the laptop and same. I did not open the laptop lid as I wanted the monitor resolution, but could try that if need be. It "feels" like a Lenovo issue but do not know. No setting I can find that would contribute to this. Ideas please? Thanks.
UP2516D and HDMI issue