I wanted to update the thread in case someone experiences this issue with their equipment in the future. I had previously enabled DMZPlus for the connected 3rd Party Router. This enables the AT&T Gateway to assign the public IP address to the router itself and (I believe) disables the firewall and NAT on the gateway just for the DMZPlus connected device. It's great if you want to use the full capabilities of your router and have servers or services that need direct exposure to the Internet.
Unfortunately, I discovered that the device for which DMZplus is enabled suffers the performance bottleneck described in the OP. Once DMZPlus is disabled and the connected device receives a private IP address, the performance is restored.
So it would seem that the root cause is a fault of the AT&T provided gateway, which as of this writing, is required for the AT&T Fiber Service (even though the box where the fiber terminates is itself a router).
Hi everyone, I have a strange problem going on with my home network. Let me explain my setup. I have AT&T fiber at 100MBPS up/down. The setup requires an AT&T router connected to the modem where the fiber optic terminates. I'm electing to not use the AT&T router because it's limited and the Wifi isn't great. So I've connected an Asus RT-AC66U AC router's WAN port to one of the LAN ports of the AT&T router.
Here's the problem. When connected to either the Wifi or the wired connection on the Asus RT-AC66U, I get 50MBPS down/120 MBPS up. When I connect to a LAN port on the AT&T router, while the Asus router is still connected and active, I get 120MBPS down/120MBPS up. Just to rule out a bad cable, I swapped them. Same result.
I have tried to power cycle all equipment, I've updated the firmware on the Asus RT-AC66U, and still this has no effect. The frustrating thing is that this issue has only appeared in the last few days and there's no real explanation as to why. All I know is that the AT&T router is putting out 120MBPS Up/Down over the wired connection; the Asus RT-AC66U should be getting that same speed and pass it along to the connected clients.
The only thing left to do is a factory reset on the Asus RT-AC66U, which I will do now. I'll report back with the result.

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