Hey fratinize,
First, we'd recommend reviewing this article for information on setting up and using iCloud Photos on Apple TV: View iCloud Photos and Shared Photo Library on Apple TV - Apple Support
If you're unable to sign into iCloud Photos, you may want to provide some additional information about what happens when you try to sign in with your iCloud/Apple ID account. Are you getting an error message?
We'd also recommend checking to ensure your Apple TV is up to date to ensure latest compatibility with Photos: Update your Apple TV - Apple Support
You may also find the information in this article about the ending of My Photo Stream helpful as well: Information about the My Photo Stream shutdown - Apple Support
"What do you need to do?
Check whether you use iCloud Photos on all of your devices. If you don't use iCloud Photos, you can save photos currently in My Photo Stream to your device. To keep your photos synced across your devices, you can turn on iCloud Photos.
Check whether you use iCloud Photos
- On your iPhone or iPad, go to Settings > your name > iCloud. Make sure that it says "On" next to Photos on each of your devices.
- On your Mac, choose Apple menu > System Settings, click your name, then click iCloud. Make sure that it says "On" next to Photos on each of your devices.
If you don't use iCloud Photos
The photos in My Photo Stream are already stored on at least one of your devices, so as long as you have the device with your originals, you won’t lose any photos as part of this process. If a photo you want isn't already in your library on a particular iPhone or iPad, make sure that you save it to your library on that device. Or, you can set up iCloud Photos.
Save photos currently in My Photo Stream on your iPhone or iPad
- Open Photos and tap Albums.
- Tap My Photo Stream > Select.
- Tap the photos that you want to save, then tap the Share button
> Save Image.
On a Mac, your My Photo Stream photos automatically import to your library.
Set up iCloud Photos
You can turn on iCloud Photos on any iPhone with iOS 8.3 or later, iPad with iPadOS 8.3 or later, or Mac with OS X Yosemite or later. After that, you can view your photos and videos in the Photos app on your iPhone, iPad, Mac, Apple TV, iCloud.com, and even sync them to a Windows PC using iCloud for Windows. "
Thanks!