I've set a router to bridge mode and I can no longer log in to the router's interface. It's connected to the internet, but I can't ascertain its IP address now.

I have two routers connected to a ER7206 Gateway WAN ports.  Both Huawei, one 4G and the other 5G. All working fine.

Each router had its IP address set via its interface, and allocated the ER7206's port a IP from the router's DHCP server.

As an experiment, I'm trialling bridge mode on the routers.

The 4G worked fine, and I could switch bridge mode on and off.  I saw a 20% internet speed test improvement.  Currently it's back to bridged.

The 5G router changed to bridge mode, and it's connected to the Internet OK, but the IP address to reach its web interface no longer works.

I can't reach it externally as mobile broadband uses CGNAT.

Where in the OC200 interface can I see a usable WAN IP address?

I found that Site > Devices > ER7206 > Details > WAN/LAN2

will give me 

IP Address x.x.x.166  When I try this IP in a browser, it takes me to the login of the ER720, not the router.

Gateway x.x.x.1     When I try this IP in a browser, it times out.

So I presume it's somewhere on that subnet.  I can probably set a static IP on a laptop and work it out, but it would be better to get good information from the OC200.