It seems that I am having an extremely similar problem to this forum post.
I have recently flashed and set up Jetpack using the sdkmanager that Nvidia provides. Initially, immediately after flashing the Xavier, I had access to internet through (I assume) the usb c connection to my host pc. However, the next day when I tried to reboot the Host and Xavier, I cannot seem to follow the forum post above to find a solution to my issue.
With the Xavier not powered up, but with the USB-C connected to a freshly booted host, run “dmesg --follow” to see what shows up as the Xavier boots. Hopefully it’ll mention a USB device and a MAC address.
On the host PC, once the Xavier boots, see what you get for “ifconfig” and “route”. Then the same thing from the Xavier.