[sxlllslgh]

It seems IPv6 NAT table is not supported now. My ISP only give me a single IPv6 address by DHCPv6, and I tried all methods I could find but no one worked.
In fact, I can get an IPv6 address with 64 bits prefix in my router like 20XX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX:XXXX/64 with DHCPv6 Prefix Delegation enabled. And if I enable radvd and set prefix to be the same as the router got (even I think it should not be set manually), my devices on LAN could also get IPv6 addresses and resovle domains, but could not ping or connect router or the Internet.
I once tried IPv6 nat on an OpenWRT router and it worked. But the nat table does not exist in the ip6tables of dd-wrt now, so is it possible to update the ip6tables version of dd-wrt?

[Per Yngve Berg]

This sound like a routing issue.

I use dnsmasq rather than radvd.

Disable radvd and put this in additional dnsmasq option:

quiet-dhcp
quiet-dhcp6
quiet-ra
enable-ra
dhcp-range=::10,::200,constructor:br0,ra-names,slaac,64,24h

Can you ping6 anything on Internet from the router?