My AppleTV is in standby and still connects to Apple servers. Why and how to switch off? I use OpenWrt as router software and see this while AppleTV is in standby since weeks:

DuckDuckGo finds

17.57.144.53 is an IP address operated by Apple
Port 5223 Details: Port used by Apple to maintain a persistent connection to APNs and receive push notifications. Some Apple applications that use this port: MobileMe, FaceTime, Game Center, APNs.

Just saying 1.2 MB is a lot traffic.
Why this behavior and how to switch off?
Why? It's checking for the above-mentioned data.
Standby does not mean 'off' on any device these days. Your smart TV &amp; cable TV box will be doing just the same sort of thing - Updating TV schedules, checking for messages from various services, checking for OS updates, etc etc.
Note: the entirety of 17.x.x.x belongs to Apple. You can really just allow the entire subnet.