What does your Nginx configuration look like? It sounds like you are using something like this for your proxy pass:

proxy_pass http://188.226.199.19:8001;

Since you've set up a firewall that is blocking port 8001, you need to make sure you are accessing it on the local host. So you want something like:

    server {
        server_name yourdomainorip.com;

        access_log off;

        location /static/ {
            alias /opt/myenv/static/;
        }

        location / {
                proxy_pass http://127.0.0.1:8001;
                proxy_set_header X-Forwarded-Host $server_name;
                proxy_set_header X-Real-IP $remote_addr;
        }
    }

You could also edit /etc/csf/csf.conf and add port 8001 to both TCP_IN and TCP_OUT

Doesn't work!!

Underlining that the url of the function is http://188.226.199.19/django/registerClient

My nginx configuration is something like this:

server {
        listen 80;
        listen 443 ssl;
        ssl_certificate /etc/nginx/ssl/bundle.crt;
        ssl_certificate_key /etc/nginx/ssl/privateKey.key;
        server_name localjob.it;
        access_log off;

        location / {
            alias /webapps/sitoweb/;
            error_page 404 = /404.html;
        }

        location /django/ {
                proxy_pass http://127.0.0.1:8001;
                proxy_set_header X-Forwarded-Host $server_name;
                proxy_set_header X-Real-IP $remote_addr;
                add_header P3P 'CP="ALL DSP COR PSAa PSDa OUR NOR ONL UNI COM NAV"';
                add_header 'Access-Control-Allow-Origin' '*';
                add_header 'Access-Control-Allow-Headers' 'Content-Type,Accept';
                add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS, PUT, DELETE';
         }
}