You have a certificate but the browser is still unhappy. Almost always this is mixed content: the page loaded over HTTPS, but something inside it is still requested over plain HTTP.
Open your browser console. It lists every offending resource by URL, usually images, stylesheets or scripts with a hardcoded http:// address.
Fix them at the source rather than with a plugin where you can. In a CMS, update the site URL setting to the https version first, then run a search and replace across the database content for http://yourdomain.com.
Once that is clean, force HTTPS for everyone. In cPanel under Domains, toggle Force HTTPS Redirect, or add this to the top of .htaccess:
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
Keep a copy of the working .htaccess before you edit it. A syntax error there returns a 500 on every page of the site.
