Let Android TV install an app from outside the Play Store
Android TV asks before installing anything that did not come through the Play Store. The permission is granted to one app at a time: you are allowing that single app to install something, not opening the whole TV up.
Begin with the first. If it works you are finished, and the other two are not needed. Go on to the next way only if the previous one did not work.
Most TVs need nothing more than the first.
1From the message on screen
When the TV tells you the app is not allowed to install unknown apps, choose Settings on that message.
A list of apps opens. Switch on the one you are installing from, for instance Simple Downloader or a file manager.
Press back, and the install continues.
Only if the first way did not work
2Through Settings
With no message to choose from, the same list can be reached by hand. Different TVs keep it in different places, so try these in turn:
Settings → Apps → Security & Restrictions → Unknown sources Settings → Device Preferences → Security & Restrictions → Unknown sources Settings → Apps → Special app access → Install unknown appsIf your TV's Settings has a search box, that is the fastest way: search for unknown.
Only if neither of the first two worked
3Switch on developer options
For when the list is missing, your app is not in it, or its switch will not move.
A few devices keep the permission hidden until developer options is on: the Google TV Streamer, the Chromecast with Google TV and some TV brands. Older Android TV boxes rarely need this.
In Settings, find About. On Google TV it is under System, on older Android TV under Device Preferences, and on some sets it is right in the main list. Whichever way you reach it, it is the same screen.
Scroll all the way down. The last line is Android TV OS build (just Build on older devices). Select it and press OK on the remote repeatedly.
The TV counts down, with something like "You are now 4 steps away from being a developer". Keep pressing until it says you are a developer; seven presses in total.
"No need, you are already a developer" means it was already on.
Now return to the first or second way: the permission has appeared.
Is this safe?
Yes. This is what you are allowing
Installing apps from outside the Play Store is an ordinary Android feature; every alternative app store relies on it.
The permission applies only to the single app you switch on. Nothing else gains the ability to install, and Google Play Protect keeps checking the device as before.
Switch it on only for apps you trust, and feel free to switch it off again once CricHD is installed.