AT&T U-verse → UVERSE and IPv6 with a cascaded router

Just got the Power tier setup and also I have all new equipment from ATT. Finally got rid of the old 2Wire and now have the NVG589.

Looking at the NVG589's firmware, I see ipv6 is enabled. Awesome! So, I proceeded to put the NVG589 into bridge mode or passthrough mode so I could cascade routers using my Asus RT-N66U. I also disabled ipv6 on the NVG589 and inputted the proper ipv6 settings into my RT-N66U.

I ran a ipv6 speed test and sadly I found IPv4 was around 45+ Mbps while ipv6 was at a measly 5Mbps. I did some reading and people recommend that the NVG589 do all the ipv6 stuff while the other personal router (my RT-N66U) does all the other stuff. This should help with speeds.

Interesting....so I tried some stuff out today. First I re-enabled ipv6 on the NVG589 and killed the ipv6 settings on my RT-N66U. I went to go run a test and to my amazement, ipv6 was no longer working or at least the test said I did not have ipv6 running even though the NVG589 did have the ipv6 setting re-enabled.

Weird. It looks like ipv6 settings do not pass through to my RT-N66U. Okay, so I turned ipv6 back on the NVG589 and my RT-N66U. ipv6 testing show it is still not working properly.

I plugged directly into my NVG589 and ran an ipv6 test and everything was green and ipv6 was working!

So I'm looking at this situation now...

1) I can plug directly into my NVG589 and get ipv6 working. (I need to test for speed still)

2) I can plug back into the switch that is connected to my RT-N66U and re-enable ipv6 settings, turn-off ipv6 settings within the NVG589, and get ipv6 working, but I'll have to deal with 5Mbps ipv6 speeds.

What am I doing wrong or is this all normal?

Retested for speed with a direction connection to the NVG589. The speed is there now. I'm getting 45+ Mbps with both IPv4 and ipv6.

So why is it I can't get my RT-N66U to handle ipv6 at such good speeds?

Because of this issue, and because ipv6 doesn't pass through to my RT-N66U, I'd have to literally get rid of my RT-N66U and just use my NVG589.

This isn't what I want to do. How can I fix this?

Did you set up the Asus router to use EUI-64 for auto-addressing on its outside interface and the configure it to use DHCP Prefix Delegation to request a /64 netblock to assign to your inside interface?

The question is what the Asus router supports with respect to ipv6. I have a Cisco 3825 running in IP Pass-through behind my NVG589 and it does ipv6 prefix delegation using DHCPv6 without any issues.

Picture of a test I ran: