I have (had ?) two Fire TV (4K max) sticks around the house. I purchased a third a couple of weeks back, but had to return it- it would go to sleep after 20 minutes no matter what. The usual hack via the Developers Options wasn't available. It seems to me the Developer Options have been blocked entirely on the newer 4K sticks.
Being a little pissed off with it, when my Prime subscription came up for renewal around the same time I cancelled it.
Last week Fire TV #1 went black. It's 14 months old. This one was prone to dropping the WiFi connection.
Last Friday Fire TV #2 went black. It's a little older.
Needless to say I've gone through all the motions of trying to refire the Fire TV sticks but they are quite dead.
The little guy on my shoulder speculates it's because I cancelled the Prime sub.
</sigh>
long live google chromecast? ;)