@Caprichoso I have a somewhat similar topology but with AT&T Fiber.  AT&T requires me to use their BGW320 gateway.  So my topology:

BGW320 - Deco BE95 - Network Switch - 10 GbE unmanaged network switch.  The BGW320 supports IP Passthrough, which has its pros and cons.  One pro is that it still provides Local IP addresses to the Ethernet ports and I connect my PS3 and Xbox360 to it.  (The goal is to achieve "Open NAT" but they are really for HD-DVD and Blueray.  They are collecting dust for the most part since the advent of Netflix).

  @Caprichoso I searched and it should not be difficult to switch the XB8 to bridge mode.  There's an app to control the gateway.  I suggest you establish connection/XB8 control via the App, then afterwards enable Bridge mode.  Once the XB8 is in Bridge mode you should continue to have access to it in the app.  This will assign the GE800 a WAN address and you can then change the DHCP to 10.0.0. subnet.

  @Caprichoso 

I had been planning to replace the RV340 when swapping the XB8 with a newer modem with 10Ge support.  Removed the RV340 from the chain to simplify things.

When the XB8 goes into bridge mode the GE800 doesn't see the XB8 on either 10 GbE port.  Totally lost internet connectivity. Had to connect my Mac directly to the XB8 and reset the XB8 via the Xfinity app to get connectivity and then disabled bridge mode.

(Interestingly enough this new configuration (XB8 with DHCP active 2.5 GbE port  > GE800 > switch > Mac Speedtest) gives me 1530 Mbps, 100 Mbps faster than when the RV340 was in the chain.)

So right now the GE800 is directly connected to the XB8 2.5 GbE port  with a dynamic IP address of 10.0.0.155.  

The Mac got its IP address from the GE800 (192.168.157) even though the DHCP server is active on the XB8, which is required for the GE800 to see it.

The good news is you can have the GE800 do the firewall function.  The not so good news is that there's some sort of subscription fee associated with it as it is provided by Norton per my understanding...

Fee is ~$70 a year for "TP-Link Tether Total Security Package (Annual)" which can be purchased from Norton or TP-Link.  It enables statistics via the Tether phone app.

So the question is (after hours spent debugging) is how to get the GE800 to see the XB8 when it is in bridge mode?

  @Caprichoso The Bridge mode is a little tricky like that, which is why I recommended setting up the phone app, (which should will be able to access and disable Bridge mode).

What happened may be related to your ISP seeing a change from XB8 to GE800 and not providing service to the GE800.  In this case one method is to clone the XB8 MAC onto the GE800, another is to contact support and explain what you are trying to do.

  @Caprichoso If I didn't already have a Norton subscription myself, I would personally subscribe to this on the router.  I renewed the subscription when it expired and Norton offered a substantial discount to renew.  (I'm not too fond of them as in the past they have asked me to renew earlier than the expiration, and started the new subscription on the date I paid).  They also make you initially set up auto renewal and it is substantially higher than their discounted/promotional intro.

But since it is a third party can't complain TP-Link/China is snooping into your network...