I also do this. Xfinity went out for a few hours earlier this month and Unifi failed over almost instantly, and within minutes we had high speed internet once I upgraded us. The standby mode would have been plenty for basic web browsing, too.
$5/mo for pretty guaranteed connectivity, plus being able to take it around with me on travels is pretty awesome.
I was paying for gigabit with the local ISP and it slowed down and lost connection so frequently I bought a Starlink (the regular one, not the mini) as a "backup."
As per the usual, my internet went down and I switched to the backup Starlink. After working with it for about a week I cancelled my ISP.
Turned out around 350MBPS down was fine for everything I was doing (and it's way more reliable).
Kinda drifting off topic, but I'm so bitter over this
My girlfriend had been paying for 1Gb fiber for about 5 years at the insistence of the rep because "You stream 4k content and use your internet for work". $110/mo or something. Verizon comes by and sets her up with a modem and an "auto-route smart 2.4GHz/5GHz" router which slots you into a frequency based on...something. Who knows because it didn't work. It just put everything on 2.4GHz.
I noticed while at her house that the internet was painfully slow downloading large files and dug into it.
For those who don't know, 2.4GHz will typically top out around 100Mbps. Around the house you're looking at closer to 50Mbps. With 5Ghz it's much better, about 500Mbps typical, but verizons awful "smart" router just put everything on 2.4GHz.
So for years she had been paying for 1Gbps, Verizon happily taking her money, while she never saw over 100Mbps. It's also not like they tell you anywhere that the router they give you will only realistically offer 1/10th your Gb speed. Such a dumpster tier company. I can only imagine there are tens of thousands being conned by this scheme.
Anyway, I put in a new router and switched to the cheapest plan. The internet is now much faster.
Also "renting" their router/modem to you is typically a bad deal. (Billing details may depend on local laws.)
Getting my own modem and router easily paid for themselves, plus I'm not arbitrarily locked out of anything.
I hate that it works so well these days. I have my antenna right out ground level between the house and trees. Absolute worst case scenario, and it's been rock solid in everything but the heaviest of rain storms for almost a year now. Still, the occasional slowdown or half-second outage really screws up Android's idiotic magic for switching between wifi and cell to the point that my pixel phone is basically useless at home. But that's more of a "google knows best" problem.
What is the role of Unifi here? I read the article and went to their site but I still have no clue.
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).
Most likely to be a router, configured to fail over.