SHIMx64.EFI not shown in BIOS
Solution 1:
What that page from the Dell website doesn't tell is that you have to go to System configuration / SATA Operation and switch from RAID On to AHCI.
Ubuntu will not be able to see the Dell XPS 13 SSD if you don't modify that option.
The rest of the installation procedure should proceed normally.
Also, note that the setting up of SHIMx64.EFI should be performed after the system installation, not before.