I have similar setup but my NextCloud installation is snap version so SSL certificate installation process may vary but for SSL certificate carry forward from cPanel, here are the steps.
In your paid hosting log into your cPanel. Navigate to your SSL page, there should be an option called INSTALL AND MANAGE SSL FOR YOUR SITE (HTTPS)
Under that you can view the private key / cirtificate and cirtificate authority in plain text
You can copy them and save in your local machine as plain text file but with .pem file extension.
These 3 files can now be used in your local server with anysubdomain.yourdomain.com
Thanks