upstream **otp_backend** {
  server localhost:**port**;
}

server {
  listen 443 ssl;
  server_name **host_name**;

  index index.html index.htm;

  location / {
    try_files $uri @backend;
  }

  location @backend {
    proxy_pass http://**otp_backend**;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection 'upgrade';
    proxy_set_header Host $host;
    proxy_cache_bypass $http_upgrade;
  }

  ssl_certificate     /etc/letsencrypt/live/**host_name**/fullchain.pem;
  ssl_certificate_key /etc/letsencrypt/live/**host_name**/privkey.pem;
}