I'm using a FTTH internet connection, where both IPv4 and IPv6 are working fine with the FritzBox I got from the provider. But I wanted to use my own device, which runs OpenWrt v21.02.1.
I had no problems using IPv4, but I was unable to use IPv6. When sniffing the WAN interface I saw, that there was no reaction to the DHCPv6 Advertise from the provider. Instead after a timeout the OpenWRT box restarted DHCPv6 with a Solicit request.
After quite some time I noticed, that the default firewall rule Allow-DHCPv6 requires a src_ip and dest_ip in the fc00::/6 address range. But the source IP of the DHCPv6 packets from my provider have a unicast address (2a01:...). After removing the
option src_ip 'fc00::/6' from the firewall rule, IPv6 starts working right away.