Problem with install on a Amazon Fire tablet

I understand that I need to side load the app on a Amazon Fire tablet because it is not available in the Appstore.
I downloaded com.bluefire_llc.apps.v11.2.2.apk but I get an "app not installed" message at the end of the installation process.  Can you help?  

It's probably because your Fire tablet is running an older version of Android that is no longer supported by the App. That's one of the reasons we no longer recommend Amazon tablets. See the Getting Started document for more information.

Getting started says "The App is supported on Amazon Fire tablets running Fire OS 7.1 API Level 26 or higher."  Mine is running 7.3.3.1.  Shouldn't that work?