You added the library via Composer, but as it sounds you have not loaded Composer’s autoloader in your PHP code. This is necessary so PHP knows where to find classes (without the need for manual require() statements for each file).
If you check the documentation, you’ll see this code
require __DIR__ . '/vendor/autoload.php';
This is what you should do in your main plugin file as one of the first steps (definitely before you use any of the classes).
So the final plugin file will look something like this
<?php
/**
* Plugin Name: ...
* Plugin URI: ...
* ...
*/
require_once __DIR__ . '/vendor/autoload.php';
// rest of your code