Fraser,
I have a BGW320-500 gateway from AT&T. I have a 1GB Fiber plan. I disabled the DHCP server on my gateway thinking it may be causing IP Address conflicts with the understanding that function was now only to be done by my router. Unfortunately, I lost the ability to connect to either the router or gateway. I had to factory reset both to be able to access the gateway again.
I obviously touched nothing on the gateway side, then I let the router set itself up the wizard. It all worked again, so I decided to attempt to redo the settings as it was before disabling the DHCP Server on the gateway. This included bandwith allocation on both the application and device side, setting up QoS with speeds confirmed to by SpeedTest to be in the low 900s on both the upload and download for the relatively simple things. I then went into the advanced settings to manually select "Use this MAC Address" for the Router MAC Address (confirmed to be the router BTW). I changed nothing in the WAN or LAN setup, and I also noticed that "Use Router as DHCP Server" was checked as expected. I went to the gateway and set the device to be in IP Passthrough with the mode being DHCS Fixed and tied to the router's MAC Address. I also disabled all WiFi on the gateway. The last change I made was in the router to the "Internet Connection Type" to be Pass Through before applying it and restarting both the router and gateway. Alas, the screenshot was my result from Bufferbloat. QoS was not enabled for the test to be clear. Any ideas?