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

FURRYe38 will look into option 2 but ISP kit is extremely dumbed-down so not too hopeful about setting up DMZ. 

CrimpOn Thanks again for the extra detailed explanation. Indeed I'm not in need of (or inclined) to access my LAN over a public IP or domain. So double NAT might be OK for me. What put me off is that when I tried it, the Orbi reacted by setting the subnet to 10.0.0.x and as I have lot's of home-brew IoT with baked-in IP's for various things running on local servers (there's around 30 reserved mappings for MAC to fixed IP to in the Orbi) and I don't want to re-flash a lot of embeded firmware. The Orbi looked like it was doing its best to honor the mappings by keeping the last byte of the IP though.

If I could somehow keep the same IPs handed out by the Orbi via DHCP then I'd be OK with this solution. I think I can change the IP address range for the ISP gateway DHCP and I did notice that it can also reserve specific IP's for connected device MACs. I just had to quickly revert to my old router/modem setup to keep the heating and lights going - so I hadn't looked into it further 🙄