Downgrading to an older version seems to address the problem
After updating their AirPort / Time Capsule firmware, Mac users are reporting that their IPv6 connection is broken and that wireless clients are not being assigned IPv6 addresses.According to Bradley Froehle writing on Apple Support Communities, attempting to set different IPv6 tunnels will not resolve the issue. Affected users can only downgrade to address the flaw.
“Reverting to 7.6.1 immediately solved the problem,” Froehle confirms.
To downgrade your firmware in AirPort Utility 6, click on the Airport you want to downgrade > hover with your mouse cursor over the version number > press Alt/Option to get a drop down menu > select the version you want to downgrade to.
There are quite a few things users should know about installing AirPort firmware updates, which is why Apple encourages users to go through the documentation first.