I know this is an old thread, however I'm having some issues with this as well. My router can access IPv6 perfectly fine. My clients are even receiving both 2001 and fe80 addresses, however I can not for the life of me get the to talk to any ipv6 based addresses. ipv6-test.com says I don't have an ipv6 address (from my client), but that my DNS is connecting to IPv6 just fine. Now my ISP doesn't support IPv6 so I know the DNS support is coming from the router. How on Earth do I get my clients to connect to the outside world via IPv6?
This is almost certainly a factor of your MTU. If you're on linux you can use tracepath to estimate your MTU. If you're on Windows, try mtupath