Could you tell us how you are using the second subnet that the ISP provided? And it might help if you would post from the config at least the parts that deal with both of the subnets? It is difficult to know if you need to add something until we know what you already have.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...