If you are using Server Side Includes SSI in your HTML files, and they are not working, you most likely need to enable them in your [.htaccess] file. Add the following code in the [.htaccess] file towards the top.
Option 01:
AddType text/html .shtml
AddHandler server-parsed .shtml
Options Indexes FollowSymLinks Includes
AddHandler server-parsed .html .htm
Option 02:
Options +Includes
AddHandler server-parsed .html