Ubuntu 18.04 Nginx - always only shows the welcome page - nginx

Question

I am using Ubuntu 18.04 default nginx configuration, with some minor changes. But I always sees the nginx's default welcome message. All config files follows:
/etc/nginx/nginx.conf:
user www-data;
worker_processes auto;
pid /run/nginx.pid;
include /etc/nginx/modules-enabled/*.conf;
events {
worker_connections 768;
# multi_accept on;
}
http {
##
# Basic Settings
##
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
# server_tokens off;
# server_names_hash_bucket_size 64;
# server_name_in_redirect off;
include /etc/nginx/mime.types;
default_type application/octet-stream;
##
# SSL Settings
##
ssl_protocols TLSv1 TLSv1.1 TLSv1.2; # Dropping SSLv3, ref: POODLE
ssl_prefer_server_ciphers on;
##
# Logging Settings
##
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log;
##
# Gzip Settings
##
gzip on;
# gzip_vary on;
# gzip_proxied any;
# gzip_comp_level 6;
# gzip_buffers 16 8k;
# gzip_http_version 1.1;
# gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
##
# Virtual Host Configs
##
include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/*;
}
/etc/nginx/conf.d/default.conf:
This file is empty. And this directory only contains one file.
/etc/nginx/sites-enabled/default:
upstream backend {
server 127.0.0.1:8068;
}
server {
listen 80;
listen [::]:80;
# server_name _;
location / {
proxy_pass http://backend;
}
}
On the server, the http server runs correctly:
$ curl 127.0.0.1:8068
some contents...
In another computer, only the welcome message shows:
$ curl http://the-server-ip-address
the default welcome to nginx message follows...
I've also tried to access the ip address in my Chrome and disabled caches, but the same welcome page always shows.
/var/log/nginx/access.log and /var/log/nginx/error.log shows no access logs and no errors.
Does my nginx config files have a problem? Thanks!
(I just tried the same config files on a 16.04 ubuntu server and it works correctly.)
I've run the follow commands to test and reload the configs:
$ nginx -t
$ nginx -s reload
updated
Sorry, I just tried some other operation:
I run the following command on this server:
$ ifconfig
and get the ip address is 192.*** rather than the ip address I used to access(ssh to) this server.
I run
$ curl 192.***:80
and the server responds correctly.
But why the ip address I used to ssh to the server always shows the welcome page?
$ curl the-server-ip-address-for-ssh:80
still the welcome page...
Thanks!