RBR850 Can't get internet from Xfinity xFi Gateway

I ask because having a customer router connected to an ISP router is quite common.  I believe that thousands of customers open the box containing their new WiFi system (Asus, Amazon eero, Google Mesh, Linksys, TP-Link, Netgear, etc.), plug the cable into their ISP router and start using it.  Many of them do not even change the default WiFi credentials.

This creates what is called a "Double NAT" condition which interferes with specific internet usage.  Certain internet gaming, internet file sharing, and accessing home devices from the internet can be quite complicated with a Double NAT. (not impossible, but more technical to set up).  If "it works great" with the XB8 in the default mode and does not interfere with something you seriously need to do, then that might be the solution.

On a lark, I deliberately set up a "Triple NAT": Orbi router -> connected to Linksys router -> connected to Orbi router. All of them in router mode.  With a computer connected to that final router, I could stream video, browse the web, do email, etc. just fine.  Setting up internet file sharing was a chore - but possible.

No one seeks out a Double NAT on purpose.

A random comment on the nighthawk experience mentioned in message 1, when disconnecting the primary device from a modem (including CM1100 and XB8) and replacing it with another device, it is almost always necessary to restart the modem.  It has already learned the MAC address of whatever it has been connected to and will need to start over to learn the MAC address of the new device.

Couple of updates. 

1) I tried taking the XB8 back out of bridge mode and the Nighthawk did indeed still work fine, but the Orbi still did not passthrough any internet.

2) I picked up a new RBR850 from the store to try to confirm for sure whether it's purely the Orbi's fault. This one connected right up (with the XB8 back in bridge mode) just fine. Works great. 

Of course I'd rather not have to buy a brand new router if there's a way to get my current one to work. It clearly is having some sort of issue. Maybe it's a bad WAN port after all. Just super weird that the Connected Devices list shows the XB8 when it's connected, so it's obviously "seeing" something on that port. But not apparently working beyond that.

I'm going to reach out to Netgear support tomorrow and see if there's anything else they can think of to do, or if this is just a bricked router at this point.

If the WAN port on the router doesn't work right but it otherwise seems to function fine, is there anything else at all it's useful for? Is it possible to put the satellite firmware on the router and just use it as a satellite? Or is the entire box junk if the WAN port goes bad?

• I am not aware of any explanation of a method for converting an AX series Orbi router into a satellite. (A method was published years ago for converting the original RBR50 into an RBS50 satellite.)
• It might be worth doing one more Factory Reset on the old Orbi router.  Connect a computer directly to the old router and then connect the old router to the current system to see if the WAN port recognizes the connection.  After a Factory Reset, that old router should come up in a "please configure me" mode.  It prompts users to use the Orbi 'app' for configuration, and I scroll down to the prompt which says, "I do not have a cell phone" so that I can use the computer that is connected to it. (What business is it of Netgear's whether I have a cell phone or not????)