There are some Dell USB to Ethernet adapters that work well for this. There are others as well. I just don’t have the specifics handy.

https://www.amazon.com/Dell-Adaptor-USB-C-Ethernet-DBQBCBC064/dp/B016FK6SCW

https://www.amazon.com/Dell-0K17480-Gigabit-Ethernet-Card/dp/B00WHW76NI

You can also search for USB Ethernet PXE.
Only certain chipsets will work.

We are currently using the undionly.kkpxe file instead of undionly.kpxe to boot Hyper-V Gen1 /bios VMs. This file is also working correctly for the physical hardware that we have tried it on so far.