I have a Mk I Chromecast that I setup using the original Chromecast application on a win10 laptop. That application still seems to work but is now unavailable on the web. What you're apparently supposed to do now is to go to this URL using Chrome.
https://www.google.com/chromecast/setup/ and then choose "set up your chromecast using this computer". However, it never finds the Chromecast. Despite the fact that I can successfully cast tabs or desktop with no problems.
If I use my android phone, it can see the chromecast device and all its details. I'm just puzzled why it never works from the laptop Chrome.
In the forums there seem to be a few other people with the same problem. But none with exactly the same setup that I can see. In any case, the help from the Google support staff is at the level of "restart Chromecast, update chrome, turn on wifi, etc, etc".
WTF?
UPDATE: I now see where you indicate it's a laptop. Have a look at your windows firewall, maybe disable it (temporarily) to see if it works.
Firewall fully off. Has no effect. Still can't find it.
I wonder if it's looking for chromecast devices that haven't been setup and so are on peer to peer wifi.
http://www.christmasquotes2017.com/2017/10/merry-christmas-funny-sms-and-wishes-in.html
I seem to recall I had issues with this when they first came out.
Turn off the 5ghz signal on the router first then try again. AFAIK the original Chromecast didn't support 5ghz.
The one thing that doesn't work is setup from within desktop chrome. It never finds the Chromcast. I'm beginning to think that this is a one time only web application that only finds Chromecasts in their initial peer-to-peer networking state. This is only an issue if you needed to do something like change networking credentials on the Chromecast and didn't have an android or iOs device available.