Use Composer Package inside class - Namespacing error

I have my theme, with a folder called classes, with a single class:

wp-content
|- themes
   |- mycustomtheme
      |- classes
         |- FooBar.php

In FooBar, I would like to use this package here: Soundasleep / HTML2Text.

And use it like this:

In FooBar.php

  doSomethingBadass( $some_html ){
    wp_mail( '[email protected]', 'test', \Soundasleep\Html2Text::convert( $some_html ) );
  }

But whatever I do, I keep getting this error:

Fatal error: Uncaught Error: Class 'Soundasleep\Html2Text' not found in /app/public/wp-content/themes/mycustomtheme/classes/FooBar.php:60

My attempt

I assume that it's something to do with the namespacing.

I'm including FooBar.php by requiring the class in the functions.php.

How do I get past this? And if possible, then please link to a documentation or a place where I could have figured this out. I've been on this for 2 hours now.

Update

I'm including Composer with these lines inside functions.php:

if( file_exists( get_template_directory() . '/vendor/autoload.php' ) ){
    require get_template_directory() . '/vendor/autoload.php';
}