Orbi (RBR50) DNS keeps failing.

Ran into this issue as well.    This Reddit thread has the best workaround I've found for the bug(s) that Netgear pushed out with its most recent release for this model.    

https://www.reddit.com/r/orbi/comments/m5l3vk/orbi_dropping_dns/

There are several options.    I think the best one is to use the debug.htm page to enable telnet, log in, and modify the DHCP settings to tell clients to contact  external DNS servers directly thus bypassing the failing proxy on the router.   This change sticks until the router is rebooted which is rare for me.  All devices will renew theri DHCP lease within 24 hours (or less) to get the new setting.    I recommend forcing devices to renew their DHCP lease to get the new settings sooner.   You can unplug the LAN connection, toggle wifi on/off, reboot a device, or issue the command to renew specific to your device/OS.    However if you have one computer plugged directly into a LAN port, one simpler way to reset all the wifi attached device's DNS config is to change the wifi name configured on the Orbi,  wait a few minutes, and then change it back.   I judged "a few minutes" by monitoring my phone to see when it decided the wifi connection was lost.   Once back to the original name, my (Android) phone took a couple of minutes to accept the wifi was stable.  

There are also reports that the bug is an interaction issue between the DNS process (dnsmasq) and the traffic measuring/metering feature, and that disabling the traffic measuring feature will avoid the bug.   Since I don't *need* the measuring feature, I disabled it just in case a thunderstorm glitches the power, there's a chance the Orbi will remain reliable afterwards without the effort/distraction of the reconfiguration workaround.