Dual LG Ultrawide Monitors on Dell 3100 Docking Station

[AlbanRampon]

Hello,

Would you please give us the logs so we can look into it (3rd link of my signature)?

The most likely reason is that the native resolution is above what the Dell D3100 can do so it doesn't offer it.
For instance, it can drive a single 4K over the DisplayPort output with 2 HD screens on HDMI. It can driver higher than HD on the HDMI, but you're most likely above spec.
To test this hypothesis, try to disconnect 1 of the two LG displays, do you get the native resolution driven? If so, it confirms you're above the Dell D3100 specifications for dual outputs because it needs the resources of two heads to drive on screen. You could work around that by connecting one of the two displays through DisplayPort. If you use a DisplayPort to HDMI converter, you MUST use an active converter otherwise it's just HDMI signals over a DisplayPort plug (roughly) and the limitations of HDMI apply.

The definite answer will be when I've got the data from the logs which will tell me what the monitor wants.

Kind regards,
Alban