defmodule AppWeb.Endpoint do
  ...

  socket "/socket", AppWeb.UserSocket,
    websocket: [connect_info: [:peer_data, :x_headers]]

server {
    listen <interface-ip>:80;

    server_name my-site.ru;

    location / {
        proxy_set_header Host $http_host;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "Upgrade";
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_pass http://127.0.0.1:4000;
        proxy_http_version 1.1;
        proxy_read_timeout 300;
    }
}

defmodule AppWeb.UserSocket do
  
    @impl true
  def connect(_params, socket, connect_info) do
    remote_ip = connect_info[:peer_data][:address] |> :inet_parse.ntoa() |> to_string()
    socket = socket |> assign(:remote_ip, remote_ip)
    {:ok, socket}
  end

  ...
end