Firestick is a tricky device to install 3rd party apps onto.
Easiest method to use the app in Post2 above, is install Downloader app from amazon, and follow the instructions for the settings required on your stick, to use 3rd party apps.
Read the info at link below (disregard the VPN recommendations as you cant use with this service).
Code:
https://troypoint.com/downloader-app/
Use downloader to get the Kokaleka apk file from the link below