Yup, that was it. Nuked everything, created a 192.168.1 network, fired it up, worked on the first try.
Deleted the 192 network, reverted the network changes right to the way it was before, everything’s still working even after logging out / back in. Must be an initial-setup-only item for authentication. Which makes sense, sort of. I should be able to use whatever network topology I like within my LAN. And I can, just not for this initial setup apparently.
A million thank-you’s, cheers. My only gripe would be the ‘not authenticated’ page make some sort of reference to these networking requirements.