Using LibreELEC is great for an old machine, and like many others I want to use that old system to then cast to other devices. But unlike a normal Kodi systems the functionality isn't exactly the same.
I eventually found my workaround, not sure if it will suit all needs.
In "Settings/Services" under control I set the "Allow remote control via HTTP" options and "Allow remote control from applications on this system" to ON
Under "UPnP/NLNA" I enabled and shared my libraries. Note my libraries also include other network files so that everything is accessible.
Then when you log into the websocket on a browser......what I did to make it simpler was download onto my android device "UPnP Explorer" from google play. This made it less of a pain to find a quick connection to the server. You could delete the app afterwards once you know and bookmark the web address. In the app when you find the system running LibreELEC and you select it, there is a button top right that is a rectangle with an up arrow. This will take you to the logging screen for remote control.
From the web interface console go into settings from the left hand bar for Chorus2 interface. Change default player to local.
Now when I select a file it plays on my Android device, there will be a cast option that will appear on the top left. I have not gotten this to work or no if it will or can . So instead you have to use the built-in options on your Android devices quick buttons from the upper pull down to either mirror or cast. This seems to cast to Chromecast and other casting apps on Android TV boxes. I'm sure it would work for smart TVs as well.
Hope this helps someone!