So your site is responding correctly to https:// access ... the icon in the browser bar shows cert comes from LetsEncrypt ... right?
If so ... still more to do with moodle.
1. edit config.php files wwwroot line ... which probably is still http:// ... obviously, changed to https://
2. Not done yet ... all the internal links in Moodle have been using that wwwroot variable to build and record in the DB for your Moodle. There is a tool to help find and replace all the now errant protocols.
** but before you use it ... *BACKUP* your database ... and know how to restore that backup, just in case.
goto: https://yoursite/admin/tool/replace/ OR depending upon version of Moodle (newer has also: https://yoursite/admin/tool/httpsreplace/
You will be prompted to login as an admin level user.
Search for http://site/ and replace with https://site/
If you do it, right ... need to also clear the server caches ...
Purge them all!