I have an RT-N66U with the now-latest firmware 188.8.131.52.376_3861, which includes the IPv6 firewall and I think an extra setting (it asks for server IPv6 address as well as IPv4 address, which I haven't seen mentioned in other posts).
I have nearly but not quite got it working, and can't figure out what I have wrong.
From the router itself, when telnetted in, I can ping and traceroute to ipv6.google.com, so it looks like the tunnel is fine. radvd is running, and various devices have picked up IPv6 addresses in the correct range. However none of my devices can ping the router (neither on link-local nor global addresses), and the router can't ping them on any of their addresses. That ought to be the easy bit!
The devices seem to be able to resolve IPv6-only domain names to addresses, but I assume they are doing so via IP4 DNS.
My tunnel details look like this:
Server IPv4 Address:184.108.40.206
Server IPv6 Address:2001:470:1f1c:222::1/64
Client IPv4 Address:220.127.116.11
Client IPv6 Address:2001:470:1f1c:222::2/64
The IPv6 log on the router says:
IPv6 Connection Type: Tunnel 6in4
WAN IPv6 Address: 2001:470:1f1c:222::2/64
WAN IPv6 Gateway: 2001:470:1f1c:222::1
LAN IPv6 Address: 2001:470:1f1d:222::1/64
LAN IPv6 Link-Local Address: fe80::62a4:4cff:fe28:1c68/64
LAN IPv6 Prefix: 2001:470:1f1d:222::/64
Any suggestions gratefully received!