Xfinity XB8 Gateway in Bridge Mode on 2Gbit plan, incorrectly changes to 1.4 Gbits down & 40 Mbits up

For anyone else who encounters this problem:

I recently upgraded my plan to 2Gbit (up and down, but up is limited by gateway to 250 Mbits typical).  I received a new XB8 gateway included with the plan.

I have my own router (OpenWRT running on NanoPi R6S) and WiFi Access points, so I set the XB8 to "Bridge Mode".  This worked perfectly for 1 week.

Then my speeds dropped to 1.4 GBits down and 40 Mbits up, and my plan listed "1.2 Gbit" on Xfinity web site.  I contacted Xfinity support 3 times via chat and phone.  The agents (and supervisor) tried to help but they had no idea what was wrong.  Finally they sent a technician to my house.

Thankfully I got a tech who knew exactly what was wrong.  XB8 gateway in bridge mode on 2 Gbit plan will get loaded with an incorrect boot file that restricts it to 1.2 Gbit down & 40 Mbits up.  They can't fix this remotely.  He installed a new XB8 and left if in the normal wifi gateway mode.

To make XB8 work like bridge mode, without actually setting it to bridge mode, I did the following:

1. Disable all radios (XB8 UI on 10.0.0.1)

2. Disable IPv6 and IPv6 Firewalls (under Custom Security on XB8 web UI)

3. Disable "Advanced Security" (Xfinity app home tab)

4. If you need port forwarding, assign your router a reserved IP address (10.0.0.3). Then in Xfinity app, add 2 forwarding rules for Port ranges 1-16384, and 16385-32768 (single rule appears limited to range of 16384), TCP/UDP traffic forwarded to your router.  Forwarding ports 32769 and above does not work at all, but I don't need that.  You may need to wait quite a long time until your router shows up as a device in the forwarding.

5. You can also set your router's DNS addresses, to avoid unnecessary DNS hop through XB8.

The above steps will make it work for IPv4.  IPv6 is more complex, and probably can't be made to work on a consumer router downstream of XB8.  I was able to make IPv6 work using OpenWRT, with difficulty, by configuring it to relay DHCPv6-service, RA-service, and NDP-proxy from lan interface to wan16 interface set as designated master.

The above steps make it work almost like the XB8 is in bridge mode.  The XB8 is still performing NAT function for a single device (my router), but that doesn't seem to hurt anything.  All incoming traffic will go to my router, either by NAT matching it to a connection, otherwise by port forwarding rule.  

This is a hacky way to do it, but it saves me from having to buy a new modem.

The tech said they have seen this problem multiple times, and it can occur on other gateways also.  I'm guessing its a system-level bug that pushes the wrong boot file.  It's an expensive bug for Xfinity - hours spent with phone/chat agents who haven't been trained on it, tech sent to my house, customer frustration.

-Kevin