Latitude 5401 and WD19 dock - Screen maximal resolution when lid closed

@Immortall  I'm not sure why the lid is making a difference here, but fyi if you're able to get QHD (correct term for 2K) and FHD running simultaneously, at least one of them will be running at a refresh rate below the standard 60 Hz.  The Latitude 5401 only supports DisplayPort HBR2 over USB-C, and as you'll see if you check the WD19's manual on support.dell.com, with that type of system you can only run dual displays up to 1080p each (technically 1920x1200, but Dell seems not to acknowledge that such displays exist anymore).  The ability to run dual displays up to QHD through a WD19 requires a system that supports DisplayPort HBR3 over USB-C, which the GPU in that system doesn't support.  So that's probably why your QHD display is running at FHD when the lid closed.

But since the Latitude 5401 comes with Thunderbolt 3, you could fix this by simply getting a WD19TB, which can tap into 4x more display bandwidth than the regular WD19 with your system and therefore can run dual displays up to 4K 60 Hz each or triple displays up to QHD 60 Hz each.

(Side note about "2K resolution": That is a dangerous term to use because so many people use it incorrectly, including you in this case, and sometimes the difference can matter for technical reasons, including in this case. 2K resolution is technically a film resolution of 2048x1080, but when used to refer to "consumer" resolutions, it actually refers to the nearest and slightly lower consumer equivalent, which is 1920x1080, aka Full HD or 1080p -- because 1920 is very close to 2000, i.e. 2K, horizontal pixels. Similarly, 4K is also technically a film resolution of 4096x2160 pixels, but when used in the consumer world refers to the slightly lower resolution of 3840x2160, aka Ultra HD or 2160p. The resolution you have is rightfully called 1440p or QHD -- or I guess you could call it 2.5K, but nobody does -- and using either of those terms would make it much clearer to everyone what you're talking about. The reason it can matter is because the bandwidth difference between 1080p and 1440p displays affects available display configurations in certain setups. For example, you had just said you had "dual 2K displays" and they weren't working properly, then if they're actually FHD that should be possible -- but dual QHD would not be possible.  But if you say "2K" meaning one resolution and someone else assumed you meant the other, you could end up going down the wrong path on troubleshooting for a while.  I only figured you meant QHD here because you differentiated from FHD.  QHD is only one extra character to type compared to 2K, but it adds a lot of disambiguation value.)