[AC87] [380.69] No WAN access with IP Passthrough from AT&T NVG599 Gateway

[Neurogenesis]

Topology:
I have FTTP with WAN port on the Gateway connected to the box on the side of the house. The AC87 has LAN 3 attached to the LAN 3 port on the Gateway.

Background:
I initially had the network setup according to the instructions here: https://forums.att.com/t5/AT-T-Inte...n-bridge-mode-or-as-close-as-you/td-p/3552057

Everything had been working fine, with everything on the same IP range (192.168.3.x) with the Gateway passing the WAN IP to the AC87. Note that the LAN3 ports are connected together, the WAN on the AC87 was left empty.

Problem(s):
About 2-3 weeks ago this just up and stopped working. I can't figure out why. I called AT&T support is worthless and didn't help in this scenario. The AC87 seems to be able to get the WAN IP, the Gateway registers the AC87 as present with the WAN IP, the device as (ON), the IP being the WAN IP (not the LAN IP) and behaving as the passthrough,...but very soon after this happens the Gateway indicates that the device is (OFF) with all of the other information being the same. Nothing in this scenario connected to the AC87 will connect to the internet, whether it is listed as ON or OFF in the gateway. Once off, I have to physically unplug the cable from the WAN port and plug it back in for the same instance to repeat itself.

Thing I have tried:

With everything on the same IP range 192.168.3.x​

1. Reset the Gateway to defaults by doing a factory reset, then followed the instructions listed in the URL above, rebooting the gateway afterwards.
2. Reset the Router to defaults by doing a factory reset, then followed the instructions listed in the URL above, rebooting the gateway afterwards.
   
3. Tried a different Cat6 cable
4. Changed the DNS servers
5. Updated the firmware on the AC87 to 380.69
6. Connected the cable from the Gateway into the WAN port on the AC87
   • Right after I plug the cable from the gateway into the router the AC97 gets the WAN IP.
     • Anything connected to the AC87 will NOT gain access to the internet. (all devices register as connected to the network, but no internet)
     • Anything connected directly to the Gateway, sans the router, are able to access the internet.
7. Enabled IPv6 on the AC87
8. Disabled IPv6 on the AC87

With everything on the different IP ranges 192.168.3.x & 192.168.2.x

1. Reset the Gateway to defaults by doing a factory reset, then followed the instructions listed in the URL above, rebooting the gateway afterwards.
2. Reset the Router to defaults by doing a factory reset, then followed the instructions listed in the URL above, rebooting the gateway afterwards.
3. Tried a different Cat6 cable
4. Changed the DNS servers
5. Connected the cable from the Gateway into the WAN port on the AC87
   • Right after I plug the cable from the gateway into the router the AC97 gets the WAN IP.
     • Anything connected to the AC87 will NOT gain access to the internet. (all devices register as connected to the network, but no internet)
     • Anything connected directly to the Gateway, sans the router, are able to access the internet.
6. Enabled IPv6 on the AC87
7. Disabled IPv6 on the AC87

EDIT 1/2 3:30p CST

I just tried something new going through the setup wizard instead of manually assigning IPs after the factory reset

With Gateway on 192.168.3.x and AC87 on 192.168.1.x plugged into WAN on AC87

• It works for about 5 minutes. Then dies. Same symptoms as above.

EDIT 1/2 3:50 CST

With the Gateway on 192.168.3.x and AC87 on 192.168.3.x plugged into a standard RJ45 slot on the AC87

• You can't go through the wizard, you have to choose manual setup..but this time I chose to choose via the wizard that I wanted to manually setup.
• It works! Sort of. The AC87 is reporting that no internet is connected, but all devices behind it are working. This means the internet LED is a solid red.

Any help in resolving this I am obviously grateful for.



I am at my wits end, any help on what else I can try would be greatly appreciated.