prod.exs

...

config :my_app, MyAppWeb.Endpoint,
  url: [host: "<YOUR_DOMAIN.COM>", port: 443],
  cache_static_manifest: "priv/static/cache_manifest.json",
  server: true,
  force_ssl: [hsts: true],
  http: [port: 4000, transport_options: [socket_opts: [:inet6]]],
  https: [
    port: 4040,
    cipher_suite: :strong,
    transport_options: [socket_opts: [:inet6]]
  ]

# Set path to cert folder
config :my_app, :cert_path, "/home/<YOUR_USERNAME>/site_encrypt_db"

# Set the cert mode so site_encrypt knows to hit live LetsEncrypt
config :my_app, :cert_mode, "production"

...