Install US TV Apps on Fire TV and Fire TV Stick

You need to add a valid US Address to your Amazon account in order to install US Apps on your Amazon devices.
This guide works for Fire TV, Fire TV Stick and Kindle Fire devices.

Find a Valid US Address using Google Maps

Open google maps and search for any hotel or restaurant. For example: Portland Oregon hotels
Just click on any hotel and use the complete address and 5 digit zip code which is available after the complete address.

Add a Valid US Address to your Amazon Account

Access Manage Your Content and Devices on the Amazon website:
Manage Amazon Device Content
Go to Settings,
Find the Country Settings paragraph.
Find the field for current country and click Change.
Enter the new address which you picked from Google Maps
Choose United States as country location and select Update.
On your Amazon device, navigate to Home > Settings > My Account.
Select Deregister.
Return to My Account and select Register
. Now, you have access to the US Amazon Store and you can install new apps.
For more info, please check the oficial Amazon guide:
Official Amazon Guide for Changing Store Location