Hi @linger,
The method of requesting a certificate that you’re using requires the server to be able to accept inbound connections on port 443, and requires you to run Certbot on the computer that the domain name is pointed to. You can’t use a different port number or run Certbot on a separate computer with this method.
Can you run Certbot on server A instead? If not, you might be able to use a different method to get the certificate.