Unifi is one of the few consumer-grade routers that supports dual WAN.

While the GUI and other polish certainly makes it more approachable then many I wouldn't really call it "consumer-grade", it's definitely into prosumer/SMB territory. And in that market there are thankfully a number of solid competitors fwiw, both directly the exact same head to head niche (Omada), more disjoint but sometimes higher value deals like Mikrotik, and open source solutions focused on embedded (like OpenWRT) or ones like OPNsense that will run on a vast array of PC hardware. Failover should be pretty straight forward on all of them, whatever is being used for routing just needs at least three network ports (2 or more for WAN, 1 or more for LAN).