Nope, if it works over Wi-Fi, it works over USBNet.
Which would just imply NM screwed the pooch. I can't tell you much about that, I've never used NM, I just throw an ifconfig at it.
I'm not at my usual box right now, so I don't have it on hand, but
the Kindle docs may help figure this out. Be warned that it's probably using a completely different subnet in the examples ^^.
The
legacy Kindle docs should be using the same subnet, though, but the docs themselves may be less verbose

.