STEP 1: INSTALLING ES FILE EXPLORER ON FIRE TV STICK
On your Fire TV, go to ‘Settings’ -> ‘System’ -> ‘Developer Options.’
Turn on ‘Allow apps from unknown sources.’
From the Amazon Fire Stick main menu, go to ‘Search.’
Type ‘ES Explorer’.
Click on the ES Explorer app icon to download and install it.
Launch the ES Explorer app.
STEP 2: INSTALL TERRARIUM VIA ES EXPLORER
From the ES Explorer menu, go to ‘Tools’ -> ‘Download Manager’ in the left-hand menu.
Click on ‘+ New’ from the bottom menu.
For ‘Path’, type https://github.com/NitroXenon/Terrarium-Public/releases/download/1.8.0/app-release.apk and click ‘OK’.
For ‘Name’, input ‘Terrarium’.
Select ‘Download Now’
The Terrarium installation file will now begin to download.
Once the download is complete, click on ‘Open file’ to install it on your Amazon Fire TV.
Go back to your Fire TV home screen.
Launch the Terrarium TV app.
Source Link : https://terrariumtvforpcdownload.com/install-terrarium-tv-fire-tv-fire-stick/