RBR50 - RBR20 and IP Addresses

The Orbi router creates a LAN subnet which is different from the WAN subnet that the router is connected to using one of the private IP numbers:

https://en.wikipedia.org/wiki/Private_network 

Much of the time, the router's WAN IP address is either:

• A public IP address (because it is connected directly to a modem, rather than a router), or
• A private IP address in the 192.168.0.x address space.

When this happens, the router creates a private subnet using 192.168.1.x

However, if the router itself is assigned an IP in the 192.168.1.x space (which often happens when the Orbi is attached to a combination modem/router, Netgear decided to have the LAN space be 10.0.0.x.  They could have picked any of the other number ranges, but for some reason they picked 10.0.0.x  This indicates that the Orbi is connected to a combination modem/router (in this case a fiber ONT/router)

The RBR20 sees "oh, my WAN side has an IP in 10.0.0.x, so I'll pick a different private IP space for my LAN side" and chose 172.xx.xx.xx

There is nothing to be gained by having the RBR20 in router mode.  If you change it to Access Point (AP) mode, then anything connected to the RBR20 will be in the same 10.0.0.x space with the rest of the network.  The reason you cannot find the RBR20 using orbilogin.net is because you are connected to the primary RBR50 system.