How to use Chromecast without Wi-Fi
You can use Chromecast without any Wi-Fi connectivity. In fact, there are a few different ways to do it.

Use Guest Mode
Guest Mode is the easiest way to connect to Chromecast without Wi-Fi. The feature was originally intended to allow guests in your home to hook up to your Chromecast without connecting to your private Wi-Fi network, but it also works if your phone can't connect to your Wi-Fi network.

Newer models of Chromecast actually come with a built-in Wi-Fi signal of their own, and when someone who isn't on the Wi-Fi network tries to cast to it, it broadcasts a PIN via an audio signal at a frequency we can't hear. This PIN allows that device to connect and stream content to the Chromecast.

How-To Here

How to use Chromecast without Wi-Fi
You can use Chromecast without any Wi-Fi connectivity. In fact, there are a few different ways to do it.

Use Guest Mode
Guest Mode is the easiest way to connect to Chromecast without Wi-Fi. The feature was originally intended to allow guests in your home to hook up to your Chromecast without connecting to your private Wi-Fi network, but it also works if your phone can't connect to your Wi-Fi network.

Newer models of Chromecast actually come with a built-in Wi-Fi signal of their own, and when someone who isn't on the Wi-Fi network tries to cast to it, it broadcasts a PIN via an audio signal at a frequency we can't hear. This PIN allows that device to connect and stream content to the Chromecast.

How-To Here