After much troubleshooting, I think I've narrowed down my IPV6 connectivity problem. My router, RT-AC66U sits behind ATT-Uverse NGV589 router which is set up for IP passthrough. IPV6 connectivity was verified via ipv6-test.com and the results came in 19/20 with solid speed test of 45m down and 5m up working as advertised. Once this was verified stable and tested at other IVP6 test sites, I decided to invoke parental control (on game stations due to their location being in the bed rooms) as soon as this was done and I tested IVP6 connectivity and my test bottomed out to 2/19. Turn off Parental Control, IVP6 is restored.