Nginx代理nodejs本机可以访问，外网显示504 Gateway Time-Out

近期在做微信小程序的开发，使用node.js的express框架编写后台代码，监听端口为4000，使用nginx代理https服务时（域名已备案，证书正确）发现在本机可以使用域名访问GET服务，但是在外网通过域名访问GET服务时出现504错误，请求解决。

 #user  nobody;
worker_processes  1;

#error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;

#pid        logs/nginx.pid;


events {
    worker_connections  1024;
}


http {
    include       mime.types;
    default_type  application/octet-stream;
    fastcgi_buffers 8 128k;
    send_timeout 60;

    #log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
    #                  '$status $body_bytes_sent "$http_referer" '
    #                  '"$http_user_agent" "$http_x_forwarded_for"';

    #access_log  logs/access.log  main;

    sendfile        on;
    #tcp_nopush     on;

    #keepalive_timeout  0;
    keepalive_timeout  65;

    #gzip  on;

    server {
        listen       443 ssl;
        server_name  19094044.nnuteam.club;

        ssl_certificate      cert/1_19094044.nnuteam.club_bundle.crt;
        ssl_certificate_key  cert/2_19094044.nnuteam.club.key;

        ssl_session_timeout  5m;
        ssl_protocols TLSv1 SSLv3;

        ssl_ciphers  ECDHE-RSA-AES128-GCM-SHA256:HIGH:!aNULL:!MD5;
        ssl_prefer_server_ciphers  on;

        location / {
            proxy_pass http://127.0.0.1:4001;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header Host $host;
            proxy_set_header X-Forward-Proto https;
            proxy_http_version 1.1;
            # for websocket
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "upgrade";
        }
    }
}