This is a way to "hack" chromecast, first enable with Mobiledata, and then disable after pairing/setup is succesfull but Casting will be still available.
Gather two devices
One must be an Android cell phone with mobile hotspot capability
The second can be any device that is capable of casting local media
open "settings" on cell phone
open "more networks"
select "mobile hotspot"
turn on "mobile hotspot"
go into "mobile hotspot" settings by selecting the 3 vertical dots in the upper-right corner
select configure hotspot
type in a name you would like to call your mobile hotspot network or leave the default name
scroll down to choose a password for this network
Now we must switch the Wi-Fi network that the Chromecast is paired to.
ensure that the second device (the one you wish to cast from) is connected to
the same network that the Chromecast is paired to (the pairing was done when you first setup your Chromecast)
open the Chromecast app (if using iOS or Android) or open Chrome if using Linux, Windows, or MacOS
From the app choose the name of the Chromscast or from Chrome select the Chromecast icon in the upper-right of the screen and then choose the name of the Chromecast
Select the name of the network you are on (this will drop-down a menu with other networks)
Select the name of the mobile hotspot network you created and enter password
This will successfully switch the network that the Chromecast looks to connect to
Now we will open LocalCast app and cast the video we desire. And lastly we will go back to the cell phone and turn off "mobile data" and leave on "mobile hotspot"
Source : https://www.reddit.com/r/LifeProTips/comments/38sb3v/lpt_use_chromecast_without_internetmobile_data_by/crxfse8