Hi,
I am the OP of this thread. Just to give an update, in short the problem has never been fixed for me. 1.5 years after I am still having the issue with my 4K monitor (B286HK) not having 2560x1440 HiDPI mode with Displaylink. I have since updated to Mojave 10.14.5 and Displaylink driver 5.2.1 but the problem is still there.
I have been connecting my 4K monitor using direct thunderbolt cable as a workaround (only using my Dell D6000 for the other 1080p monitor I have and the USB keyboard/mouse).
Haven't been following the thread since a year or so ago, but I was curious if this has been fixed but unfortunately no
Don't get me wrong, I think Displaylink is a useful feature. but I really cannot recommend it for 4K monitors on MacOS given the current lack of support for scaling 4K->1440p.