I think this might be because you have a kernel version higher than what the pre-built driver works with. I am using the 5.13.9 kernel right now and monitors aren't detected (everything else works though). From what I can tell from their github evdi repo, 5.12 kernel support was added in March, but not sure if it was part of the evdi version that is shipped with the 5.4 display link driver.
Ref:
https://github.com/DisplayLink/evdi/commits/devel