D8: How to move from HTTP to HTTPS when site completely built?

I have a site working fine, but it is defaulting to HTTP.  When I add the .htaccess file with the necessary:

RewriteEngine On 
RewriteCond %{SERVER_PORT} 80 
RewriteRule ^(.*)$ https://websitename.com/$1 [R,L]

only the home page works.  Every other link says:

The requested URL /xxxxxxx was not found on this server.

When I remove the .htaccess file and try to refresh, nothing works at all – not even the Admin login that was open in another tab/window.

After restoring the code and database from backup and verifying that everything works OK, I tried installing the Protocol Relative URLs module and clearing the cache.  Exact same results when doing all of the above.  There is absolutely no documentation for this module, but I get the impression that it only works if installed before creating any content – it doesn’t correct an existing website.

So how does a person make an existing Drupal 8 website work with HTTPS?

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/22/feed