After you've issued a SSL certificate in cPanel under 'Lets Encrypt', your website will not automatically always use SSL/HTTPS, it will default to using HTTP only.

If using Cloudflare then skip to the 'Cloudflare' section.

To force your website to use SSL/HTTPS, go to the 'Domains' section in cPanel and toggle 'Force HTTPS Redirect' for the domain you want to force HTTPS for.

If the domain is an addon or parked domain then you will need to ensure you have a Lets Encrypt SSL issued for its paired subdomain that cPanel creates and enable 'Force HTTPS Redirect' for this instead e.g. if the addon domain was addondomain.com then the subdomain you need to enable 'Force HTTPS Redirect' on would be [addondomain].[primary cPanel domain]

It may take a few minutes for the force HTTPS Redirect to become active, ensure you check in Chrome incognito to avoid cache.

Cloudflare
Visit the cloudflare dashboard and click on the SSL/TLS tab, then change "Your SSL/TLS encryption mode" to Full (strict). This will also fix any redirect loops you may see.