[1031982]

What happened is I had IPv6 working with ATT U-verse, however, last year after a DD-WRT update it stopped working.
At the time, I was occupied with other things and assumed it was a bug with using the beta firmware.
I now no longer think it was the update that did it as I keep a backup of firmware and configurations that work and restoring them has not restored IPv6.
I went to the extent of every time I change something, I have a text document with my configuration, and add notations when I make changes with what firmware revision I was using to back-track any issues.
Nothing showed up, and manually re-configuring the router did not restore IPv6.

I doubt it's important, but just in case, I'm actually subscribed to DSL-Extreme, and they lease through ATT by me.

Now then, as for my setup, I was given a Pace 5268AC gateway. I disabled as much as possible (WiFi, there firewall, etc), and then configured a single DMZ device that my router connects to.
I left IPv6 on, and just verified that from the gateway I can ping IPv6 addresses.
The gateway forwards the public address to my router.
The gateway has been given a static IP that I can access through my network without the need to change things.

Nothing on the gateway has changed from after I originally configured it.
I actually went though, logged my settings, and then reset the gateway to re-configure it.


Now then, on my router I have the fallowing settings under IPv6:
Type : Native IPv6 from ISP
Prefix Length : 60
MTU : 1472
DHCP6c Custom : disable
DHCP6s : disable
Radvd : Enable
Radvd custom : disable

This was working, and upon checking the gateway I should have the proper settings.

Any advise?

[1031982]

After so long, I figured it out. At some point an update was pushed to the gateway that messed up the configuration.
The solution was simply resetting the gateway!
Upon resetting the gateway, I got IPv6 back through DD-WRT with default settings after choosing DHCPv6 with prefix delegation.

I want to note that trying DHCPv6 with prefix delegation before only gave me an unstable connection.