How does NicheStack recognize Ethernet interfaces?

OK I think you probably understand enough of how it works, and you're just running into a very specific problem (which I don't know the solution of).

Keep searching for LAN91C111 related topics, or if you feel like fixing it yourself, the first steps would be stepping through alt_iniche_dev lan91c111() as that is where it should have been adding itself to the array.

Skimming that code, one thing I'm seeing is that this code will dtrap() and not add itself to the array if it fails reading a LAN91C111 register, which may be your problem.

Good luck.