To cast from one device to another they both have to be on the same WiFi network.
Cast from your computer:
Open Google Chrome
Click the three dots in the top right corner.
Click Cast...
Choose the device you want to cast to.

Cast from your Android device:
Open the Google Home application
Choose the unit you want to cast to.
Click Cast my screen -> Cast screen.
Read more https://support.google.com/chromecast/answer/3228332