After way too long trying to figure out why my phone was able to find and cast to a Chromecast device on my local network but the Chrome browser running on my mac desktop and laptop were not, I came across this "Experimental" setting:
chrome://flags/#media-router-cast-allow-all-ips
By default now, if your network is not using private addresses typical of home routers (such as 192.168.x.x) or corporate users (such as 10.x.x.x) and instead using an internet routable public IP range, then Chrome simply won't find it. If you enable this option to allow it to communicate with non-private address spaces and try again (you might need to restart chrome) then it will see and cast fine.
This is a change in default behaviour that Google introduced at some point in the last year or two because it wasn't a problem in the past.
