Is it possible to connect to chromecast through router while still using data network for internet? Here's the setup I am envisioning:

Chromecast and android phone both connected to a WiFi router/hotspot (has x amount of data usage). My phone however has unlimited data usage and I would like to pull external content (soundcloud/netflix/youtube/etc) from my phones data network while letting the Chromecast pull data using the hotspot network.

I would be mirroring my display to the Chromecast so the Chromecast should only have to get very minimal amounts of data.

Is this possible? How can I do it? I've tried even hooking both the Chromecast and the phone to an isolated wireless router (no internet) and unfortunately my phone does not fall back to using data.
I've been able to set up a hotspot on my phone and use a third android device to pair the chromecast to that hotspot. After that you can screen cast to the chromecast no problem. (ignore the wifi not on part - it will work anyways)

The third device won't be necessary after the initial setup process.