When you connect the Apple TV to any monitor, it automatically tries to select the correct resolution. In case of failure, you could get stuck with one less than the ideal image quality. See how you can quickly change the Apple TV screen resolution.
The first thing you need to do is check the optimal resolution for your monitor. This usually refers to the product listing on the screen manufacturer's website. If it is not there, it will refer to the box in which the screen was packed. In some cases, after connecting it, a dialog box with the correct resolution will appear on the screen.
Once you have determined the correct screen resolution, open the "Settings" application on your Apple TV.
Then select "Video and Audio".
Go to "Format".
This page displays some standard screen resolutions, such as 4K HDR 50Hz, 1080p SDR 60Hz, etc. Here, you can select the resolution for your screen.
The only problem is that these are not exact values and may not match the recommended resolution for your monitor. To do this, go to the bottom and select "Other Formats".
Here you will find the exact screen resolution values, such as 2560 × 1080 HDR 60Hz, along with other resolutions not mentioned in the previous screen. Choose the right one screen resolution.
If Apple TV manages to switch to the correct resolution, it will display a message stating that it was able to successfully change the resolution. Select "OK".
If the transition to a different resolution is not successful, you may see a black screen for several seconds and your screen might even turn off on its own. Press the Menu button on the Apple TV remote control to return to the original resolution, and then you can try to change it back to the compatible resolution.
Source of information: howtogeek.com