So, I just set up a brand new VPS, installed LAMP, all systems and services working.
I've tried almost everything I could find here on Stack Overflow and other guides, ranging from adding things to Apache's default.conf, to including almost every possible directive in .htaccess known to man (though I hear I technically don't need to use .htaccess, since I have root access?).
I do know that .htaccess is working because I can successfully used DirectoryIndex.
Not sure if relevant, I am using PHP 7.0.
If anyone needs more specific information, I can quickly find it. I really want SSI to be working!
Thanks!