If you are using Chromecasting or Airplay to cast to your TV from the VidAngel mobile app, both the TV and mobile device 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 Roku go to Roku Support
Roku can only Airplay with Apple mobile devices; Androids won't work.
A great way to confirm that both devices can "see" each other is to use Roku's mobile app with your Roku (the mobile app acts as your Roku remote). The app is free in the Google Play and Apple Stores. You can see how to do that here: How to use the Roku mobile app as your remote
NOTE: If you have a Dual-Band Router some Roku devices cannot connect to both networks; they can only connect to the 2.4 GHz network.
BASIC CHROMECAST DONGLE
To see how to check the WIFI network on your Chromecast go to Google Support
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