Angular application CORS issues using Nginx

Question

My Angular application is unable to make requests to my backend. This is the Nginx configuration

server {
  listen 0.0.0.0:443 ssl;
  listen [::]:443 ssl;

  server_name my.site.io;

  ssl_certificate /etc/nginx/certificates/my.site.io.crt;
  ssl_certificate_key /etc/nginx/certificates/my.site.io.key;

  root /var/www/my.site.io;

  location / {
    try_files $uri $uri/ /index.html;
    add_header 'Access-Control-Allow-Origin' '*' always;
  }

}

However even if the Access-Control-Allow-Origin is added to the header responses, the status code will be 304 Not Modified. I also tried to add the configuration suggested here: https://enable-cors.org/server_nginx.html without any success.