AirPort / Time Capsule owners experiencing an IPv6 Tunnel Error after installing firmware 7.6.3 in router/NAT mode should be glad to learn that Apple has finally posted an official fix / workaround.
Resolving the IPv6 Tunnel error after upgrading to firmware 7.6.3 in router/NAT mode seems to require a bit of patience on the user’s end, as Apple’s KB article HT5656
lists numerous steps and warnings.
In the most common scenarios, affected users will be required to obtain the correct settings from their IPv6 service provider and enter the IPv6 WAN address, IPv6 Default Route, Remote IPv4 Address, IPv6 Delegated Prefix, and IPv6 LAN Address into the necessary fields (as shown in the screenshot above).
Another set of instructions is then provided. Users must ultimately enter the IPv6 WAN Address and other information (such as the Default Route) received from the service provider. Apple clarifies that “All boxes must be filled in correctly.”