connecting RBR50 to ISP supplied modem/router using layer 2 bridge?

Your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT
https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT
Couple of options,
1. Configure the modem for transparent bridge or modem only mode. Then use the Orbi router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router. https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop
Try option #2 first...
https://kb.netgear.com/000061277/Which-features-are-disabled-on-my-Orbi-router-when-it-is-set-to-AP-Mode

AdrianM wrote:

Now I'm puzzled because I thought OSI model layer 2 network bridges acted as transparent connections between Ethernet segments. I thought they learned the MAC addresses of devices on each segment and forwarded traffic between them based on MAC. This way, all devices on both segments appeared to be on the same local network, without the need for any NAT or routing. You seem to be saying there will still be double NAT when using such a bridge?

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Yes, a layer 2 bridge is equivalent to an Ethernet cable.  It has no knowledge of layer 3, which where IP addresses exist.  When the Orbi WAN port makes a DHCP request, the upstream router returns information:

• Use this IP address.
• Use this subnet mask
• The Gateway IP for any destination not on this subnet is MY IP ADDRESS.
• The default DNS IP is MY IP ADDRESS.

Both the Orbi WAN port and the upstream router are in the same IP subnet.  It doesn't matter if they are connected with a physical cable, a layer 2 switch, a MoCA connection, a PowerLine connection.  They all appear to both ends of the connection as an Ethernet cable.

This kind of sinks the idea of putting the Orbi into AP mode. (Lack of network separation).

I appreciate the fixation over Double NAT. What is not clear is if your internet use is affected by Double NAT.  Thousands of customers simply unbox their new WiFi mesh system (Google Mesh, eero from Amazon, Asus, TP-Link, ... and Netgear), plug it into whatever thingy their ISP installed, and "off they go".  In my neighborhood at least, a significant number of them do not even change the WiFi credentials from what was printed on the label.  They have a Double NAT. Don't know what that is. And don't care.

Do you:

• Want access to LAN resources from the internet (access media server when not home? operate a public web site? want to view security cameras without using the vendor "cloud"?)
• Want to use a VPN tunnel to reach the Orbi router when not at home?
• Do internet gaming that requires you to forward ports through the router so that your home computer can at as a gaming server?  i.e. the game will not work unless you forward ports?

All of those things are possible with a Double NAT, but are more complicated to set up and manage.