@mm-Ekimia May I point you to a topic in the iPXE forum where one of the main developers was saying that “USB support is starting to arrive in mainline iPXE” only about a good year ago. They have made progress I am sure! We have seen more and more people successfully booting their devices with USB NICs on iPXE.
The biggest problem from my point of view is if you want to PXE boot from the USB NIC! Using an USB NIC with linux (as you do right now) is fairly easy because linux takes control and provides full access through own drivers. Completely different if you want to PXE boot from the USB NIC. Then your client machine - its BIOS/UEFI firmware - must be able and willing to talk to the USB NIC for PXE booting. Sounds like you don’t aim for that but just wanted to point that out!
Back to your initial question… booting iPXE via USB key - which is easier than USB NIC PXE booting but still harder than booting linux to use the USB NIC. As George already said it’s a matter of finding a USB NIC adapter that is supported by iPXE. I started a list of supported USB NICs some time ago. Unfortunately I lost track of it all as it was very hard to follow without having my own devices for testing. Maybe this is a good point to start again. Searching the forums we should be able to find a lot of information as lots of people have tried USB NICs lately, known working e.g. ASIX AX88772C…
But that’s still not the whole story I’m afraid! iPXE provides native drivers (should work both for UEFI and legacy BIOS) as well as UNDI (for BIOS) and SNP (for UEFI):
undionly.pxe, kpxe, kkpxe, usb, iso - BIOS no native drivers
ipxe.pxe, … - BIOS with native drivers and UNDI interface
snponly.efi, usb, iso - UEFI no native drivers
ipxe.efi, … - UEFI with native drivers and SNP interface
smsc95xx/intel/realtek.XXX - “single” native driver
So you might need to play with the different iPXE binaries to see which one is working for you. Please let us know which USB NIC exactly you have - make and model!!
Edit: I just updated the mentioned wiki article. Hope this is useful for a lot of people. Please keep us up to date with USB NIC adapters you try.