Not easy at all
Before started the process to issue Let's encrypt certificate, "Hello World" was shown by entering http://Server_IP. Even http://domain_name could return the same. (Off course, a browser warns the connection is not protedted but anyway "Hello World" was shown.)
However, once I sarted the process, it failed due to a 404 error. Then the 404 error became the only result when entering http://Server_IP, http://domain_name, and https://domain_name.
If the process runs successfully, maybe the setup for SSL and the server should be easy.