Hi,
I've just set up a RUTXR1 as the main router in my setup, acting as a PPPoE client for my fiber modem which is in bridge mode. It gets a WAN IP just fine, just like my Unifi gear did before it.
My issue is that when the wan interface has failover enabled, it randomly fails ping checks and goes offline. I'm attaching a screenshot of my configuration, but it's been configured with default parameters, and Google's as well as Cloudflare's DNS servers, with a reliability of 1. Unfortunately 3-4 times a day, it'll randomly decide those are all unreachable and drop my Internet connection. I never had such issues before with other gear, it seems to have appeared only on this setup.
Additionally, and I don't know if this is related, but for a short amount of time after completing my setup, while testing for connectivity on my laptop, I was able to browse the web just fine, but all outgoing pings failed. This seems to have resolved itself after a while, so I never got to the bottom of it.