Chromecast screen mirror with wifi, but no internet

[AC Question]

I currently do not have home internet (long story) but I do have a WiFi router. Now that Netflix lets you download shows, I download them while I am at work. I want to use my phone (LG G4) to cast my screen and audio while "watching" the shows on my phone. I can set up the chromecast, and connect it to my WiFi router (that is not connected to the internet) up to the point that it says the device is ready to cast in the devices window. So theoretically, both the phone and chromecast are on the same WiFi netrwork, but with no internet. But when I go to cast screen/audio, the Google Home app is saying "No compatible devices were found on your network. Any ideas?