RBR50, RBS50, AP, Ethernet Backhaul - Advice please?

You can use current FW for the system. It should be using v210 if your keeping the system up to date. 

You can review this for setting up eithernet back haul:

https://kb.netgear.com/000051205/What-is-Ethernet-backhaul-and-how-do-I-set-it-up-on-my-Orbi-WiFi-System

I would do this while in the same room as the router for ease of initial setup first. I would recommend setting up a satellite 1 at a time, connect it wireless first. Then connect the LAN cable between the satellite and router. Wait up to 5 minutes for the top of the satellite to turn BLUE. Do a IP address reservation for the satellite ON the router. Save and apply the reservation. Turn off and place in remote location. 

Do this process for each satellite you are adding. 

You should be good to connect to a unmanaged switch in between the satellies and base AP. 

If you updated to recent FW v.210, try enabling Daisy Chain. Some have mentioned that this seems to be working in reverse order, enabling means disabled actually. I would check mark this feature before you begin to add satellites. If you seem to run into problems, come back to this setting and remove the check mark.