Swipe down from the top of the screen and press Settings. Scroll down to the Personal section and press Security. Under Advanced, turn on Apps from Unknown Sources. You’ll see a warning about being responsible for all damage to your Fire that unknown source apps can do. This would be a good time to mention that while this is an easy process, you do it at your own risk. Press OK.
Refer: https://www.laptopmag.com/articles/how-to-sideload-apps-on-the-kindle-fire
Use a Chrome extension to find the APKs you want to use. I used X-plore File Manager to install. I had to add that application as having security to install from above.
Chrome Extension
Google Files APK
Refer: https://apkcombo.com/files-by-google/com.google.android.apps.nbu.files/download/apk
I uploaded the APK onto the shared drive and then installed with X-plore File Manager, by just clicking and choosing install.