Here again your favorite Random Dude.
Today's topic is about something I came across while working with an older Dell models, Optiplex 990. When running BIOS version A20, it presents the UEFI "Onboard NIC" boot option hidden.
Here is the step by step way to enable it. (Source)
Go to the BIOS of the machine (generally F2 while post is presented)
Change the boot list option to UEFI
Also place a check mark on Onboard NIC
Next click on System Configuration
Select the option Enabled w/PXE
Then go to SATA Operation
Select the option AHCI
Save the settings
After following those steps the machine should be able to perform UEFI PXE boot.