On Mac Mojave, Displaylink does not support HiDPI resolutions. These are screen modes that render the desktop at double the size so that fonts become more crisp. For example on my 1440p monitor, there's a 720p HiDPI mode. When I connect the monitor directly to my mac, this mode can be selected. When I connect via my Dell D6000, nothing happens when selecting this mode.

You can use the following open source application to select resolutions:

https://github.com/chris1111/RDM

A related issue is that custom resolutions added by the above application do not work when the monitor is connected via Displaylink - but do work when the monitor is connected directly.