If after installing the certificate you still get the message "Your connection to this site is not fully secure".
This typically happens when a website shares mixed content. Mixed Content is content on a secured site that is not secure. For a secured/encrypted website; its content such as text, images, videos, objects, scripts, link, iframe, etc… that is being delivered over HTTP instead of HTTPS.
This test will help you to find out if you're affected: https://gf.dev/mixed-content-test
Once you runt the test, you will see the content that is being loaded over HTTPS