AT&T U-verse → Re: NVG510 - Doesn't Work Anymore - Fix or Replace?

I fixed the problem, but found defects with the NVG510. I have IP Passthrough working again.

It was two things:
1. Starting May 29th using IPv6 killed your connection with IP Passthrough. So IPv6 has to be turned off (sucks if you actually want to use IPv6 and IP Passthrough)
2. The assigned subnet mask of 255.255.255.255 stopped working and you have to use 255.255.0.0. Which means you can only use Static IPv6 passthrough and DHCPS IP Passthrough does not work.

I used IPv6 and when the connection died I tried the 255.255.0.0 and it did not work. So I contact ATT.
ATT ending up providing no real answers, they sent me another modem which did not work, they said I needed new firmware which would not update.
I will admit the Tier 2 tech keep in constant communication with email, did not give up, and was willing to keep escalating this if we could not get it to work.
The final thing he suggested was to follow a customer post on their ATT forums which said they used 255.255.0.0 and static IP passthrough.
I know the NVG510 uses an incorrect subnet mask of 255.255.255.255, but that is what the modem assigns and it did worked from Feb to May without problems.

So the only common things for everyone who got it working was they had a 255.255.0.0 subnet mask when the modem automatically assigns 255.255.255.255 if you use DHCPS IP passthrough. Which means no one can get DHCPS IP Passthrough working.

I turned off IPv6 and set a static public IP address with a 255.255.0.0 and it did not work.
I rebooted the modem, then my modem would not authorize.
I rebooted again and it started to work.
I then tested/tried every permutation of settings for IP passthrough with and without IPv6. I also double check connectivity with each permutation with the modems domain 192.168.1.x. The 192.168.1.x internet always worked, but the public ip 108.x.x.x would not always work being passthrough.

The only one that works is IPv6 off, and Static IP Passthrough with a 255.255.0.0 subnet masks and even then it may require some reboots to get it to work.

So that means the NVG510 is defective and is not programmed correctly. I will still try to escalate with ATT to see if they can open a trouble ticket with motorola to fix the subnet mask for IP Passthrough and the IPv6 problem with the modem.
This is pretty pathetic for ATT and Motorola. This is clearly an amateur mistake in their programming.