Thanks for reaching out. If you're not familiar with your router settings, you may refer to your router documentation to learn how to disable AP isolation, or you may reach out to your router manufacturer for further help. Once this setting is already disabled, please reboot your router, mobile phone, and Chromecast then attempt to set up your device.
Just checking in to make sure that you've seen my response. Please let me know if you have other questions or concerns as I will be locking this in 24 hours if I won't hear back from you again. Feel free to start a new thread and we'll be happy to help.