Second external monitor strange behavior for Dell E6420, Windows 7

Question

I have Dell Latitude E6420 with Dell E-Port Replicator PRO3X (on the picture at the bottom).
I have two Lenovo L24q-10 monitors with maximum resolution of 2560x1440 (QHD).
One display (main) is connected via Display Port (DP) cable, other is connected in two similar ways over DVI-D (Dual Link) port: either DVI-D cable to DP, or DVI-D adapter to HDMI. HDMI supports QHD.

However, after PC boot, the second monitor stays in stand-by mode. If the resolution is set to the 1080p (1920x1080), it works normally.
Windows didn't offer higher resolution, but I have added QHD via Nvidia Control Panel (after drivers update, second monitor appeared normally).

I have found a way to turn it on: go to Screen Resolution (Control Panel\All Control Panel Items\Display\Screen Resolution) on the second monitor, change resolution to 1600x900, click to the button "Apply", and after Windows asks to keep this settings, I click to the "Revert". After that, second monitor turns on and stays active until reboot.

My question is: why this is happening? Why the second monitor goes to the stand-by mode after PC booting? I have the latest BIOS update (A25) and latest drivers from Dell site.

Answer 1

I did a quick google on the supported resolutions of your port replicator. It looks like only the display port can provide the resolution that you want.

https://www.dell.com/community/Laptops-General-Read-Only/max-resolution-dell-latitude-with-e-port-replicator/td-p/4453555