If you are casting to your TV from the VidAngel mobile app, they will both need to be on the same WIFI network so they can "see" each other. Make sure both devices are using WIFI and not an ethernet cable. If you have a dual-band router, make sure both devices are on either the 2.4GHz or 5GHz bands.
Confirm the Device's WIFI
To see how to check the WIFI network on your Apple TV go to Apple Support
To see how to check the WIFI network on your Fire TV or Firestick go to Amazon Support
A great way to confirm that both devices can "see" each other is to use Fire TV's mobile app with your Fire device (the mobile app acts as your remote). The app is free in the Google Play and Apple Stores. You can see how to do that here: Use Your Mobile Device Like a Fire TV Remote
To see how to check the WIFI network on your Chromecast go to Google Support
Samsung Smart TV
To see how to check the WIFI network on your Samsung Smart TV go to Samsung Support
Mobile (phone or tablet)
To check the WIFI network for your phone, you'll have to search the specific steps depending on your device, but it's usually in your phone's Settings.
If your TV device and mobile are on the same network, but they still can't "see" each other, go here for troubleshooting help: Trouble Casting