First, make sure your Apple TV is running tvOS 10 or higher. Then:
Heads up: you will see the app listed as “DIRECTV STREAM”, but don’t worry, that’s the correct app for your service.
Important: Accessing the DIRECTV app on your Apple TV will also be limited to your Home Wi-Fi network. You can change your home Wi-Fi network up to 4 times within a 12-month period by relocating your Gemini (DIRECTV) device.