How to use library in CI4?

I am working on a website with PHP 7.4.15 and CI 4.1.1. I need to use phpspreadsheet library so I installed it by composer.

but when I try to use that class , the below error shows.

Class 'PhpOffice\\PhpSpreadsheet\\Spreadsheet' not found

According to CI4 documents, CI4 find Composer Auto Load files. Do I need to set other things?

Any ideas can be helpful for me.