Back in 2017, when I got my first Android TV device (original Xiaomi Mi Box or MIBOX3), I made a bug report at Issue Tracker website on 09/21/17 because the device didn't "wake up" by using the "Chromecast Built-in".
On the next day, they replied by closing the report as " Won't Fix (Intended Behavior)" along with this message:
"Thanks for reporting the issue. This feature is not officially supported by us yet and our team is working on it. However, there are few OEM's support from their side. We are not sure whether Xiaomi Mi Box supports or not. We would recommend you to check with Xiaomi Mi Box technical support team on this"
It's been 1080 days, and the "Wake on LAN" feature is still not widely available on ALL Android TV devices, so, what I request to all of you is to go that Issue Tracker report to put more pressure on Google about this so needed feature. Remember, the more people reply, the merrier.
Thanks in advance.
Google will most likely never fix this. Install Wakelock . This will fix your problem