The Stones System
Update: July 2021: now it seems I need to activate the "Connect" app, the phone won't automatically start it anymore. So, start the connect app, then cast from your phone.
March 2021: I updated Windows 10 to build 20H2 and.. suddently casting didn't work.
There are 2 solutions: Re-install Wireless Display or Rollback update
After searching the Internet for solutions, including turning off Firewall (no thank you) I finally discovered that the Wireless Display had uninstalled itself.
Going to Project or Connect will not solve anything. The offending screen is found by going to from the bottom right icon on the system tray and then "All Settings", then click on "Settings", then on the left, scroll down to "Projecting to this PC".
Then you may well see the message that says you can't project because something is not installed and your previous settings are greyed out. You need to click on install and type "wireless display" into the box, it should come up as soon as you type "wire". Then install it!
And that's all. After this is done, it will say "Launch the Connect App", which you don't need to do, normally just press cast from your device and it should work. Note that if your computer does not have wireless or bluetooth, you will not see any options, it is simply not possible to use screen mirroring.