Since then, I can only log in to the router via wifi. Both web GUI and the telnet CLI work. I *can't* log in via an ethernet connection: the web GUI times out, and the telnet CLI doesn't offer a login prompt (though I see with tcpdump that it does complete the TCP connection.) I can ping the router OK from the ethernet connection.
I've compared the current configuration/user.ini with an old version and there are no obvious differences. What should I be looking for?
Relatedly: are there any recommendations for router (with or without integrated VDSL modem) that can handle IPv6 tunnels OK?