@Solo509 Best if you take a picture of the exact error on screen and post here.
From what you wrote so far I would imagine that iPXE is not having the right driver for your USB-to-Ethernet or USB-C adapter. Which iPXE binary do you use? undionly.kpxe, ipxe.efi, snponly.efi, intel.kpxe, realtek.kpxe?? Take a look at /tftpboot/ on your FOG server to see all the different ones you can try.