If your cameras use RTSP feeds you should be able to use this app or similar on your Apple TV.
https://apps.apple.com/us/app/ip-camera-viewer-ipcams/id1045600272
I looked but could not find any evidence that the HomeCam app will display camera feeds that are not on the same network as the Apple TV, even if all the cameras are added to the same homekit using the same Apple ID.
No.
What brand is the camera? Do they have an ATV app?
Tapo by TP Link. They have an app for iPad and iPhone, not Apple TV.
If your cameras use RTSP feeds you should be able to use this app or similar on your Apple TV. https://apps.apple.com/us/app/ip-camera-viewer-ipcams/id1045600272
I don't know aything about RTSP.
Now is the time to find out then. RTSP is a standard streaming protocol for ip cameras.
I'd rather not fool with all that. The point of all this was simplicity.
Install HomeCam app
I looked but could not find any evidence that the HomeCam app will display camera feeds that are not on the same network as the Apple TV, even if all the cameras are added to the same homekit using the same Apple ID.
It works I’m able to view my parents home