I managed to resolve this by factory-resetting the router and manually configuring it, instead of restoring a backed-up config.
It's been years since I've had to do that but I guess some cruft does build up from years of upgrades. In any case, I'm happy to be back on IPv6.
Hi, my RT-AC66U with firmware 380.57 has a working IPv6 connection with my ISP via 6rd. Router Advertisement doesn't appear to work, despite the logs saying it's enabled.
I've run Wireshark on a few LAN hosts and don't see any RA packets on the network. Hosts are not autoconfiguring...