I've spent days doing this so i guess it can be useful to somebody. I share here the steps that led me to install Ubuntu (first 18.04 and then 20.04) on Dell Inspiron 15 5593 along with Windows 10 (the unique OS after initializing the machine i've bought). I've made two Ubuntu installations cause i needed the dock DELL D6000 to work properly (its software has many many problems).
Ubuntu installer can't find any disk on Dell XPS 13 9350 (90 likes answer)
step 2 (option 6a was chosen)
disable secure boot enter in Bios (F12) and disable
I'm not sure if this solved anything but i've also followed this post advice. https://phpboyscout.uk/dell-displaylink-d6000-ubuntu-18-04-issues/
I've resized windows hard-drive space and expanded linux (ext4). It was in fact installed with 20Gb space.
There was a lag + ethernet problems on linux 18.04 when closing the lid of the laptop. DisplayLink Service suggested to upgrade Ubuntu to 20.04.
Upgrade Ubuntu 18.04 to 20.04 https://www.linuxtechi.com/upgrade-ubuntu-18-04-lts-to-ubuntu-20-04-lts/
reinstall DisplayLink drivers for 20.04 (no revert) https://www.displaylink.org/forum/showthread.php?t=67148
<< it works! >>
Now time to setup the new ethernet port of the dock (insert the same IP, mask, DNS you were using for the main laptop ethernet port) I've followed some of these steps as well https://support.displaylink.com/knowledgebase/articles/613455-how-to-configure-displaylink-ethernet
<< it works!! >>
PS: everytime you'll update your Ubuntu the lagging problem will come back. So everytime you'll need to reinstall the DisplayLink drivers and reboot your computer.