I got things to work without Phoenix and only with plug as a dependency – I just had to rework some of the responses, e.g.

import Plug.Conn

def send_error!(conn, msg) do
  conn
  |> put_status(:unauthorized)
  |> Plug.Conn.put_resp_header("content-type", "application/json")
  |> Plug.Conn.resp(400, ~s({"status": 400, "message": "#{msg}}"}))
  |> halt
end

I just needed to look past the syntactic sugar of all the functions that are available when you import Phoenix.Controller

What plug package? Do you mean master_proxy package? I suggest you do something like acme_bank example. I am running something similar to that in production and working just fine. Your main application router which delegates the the sub apps will be called through their specific endpoints. Here is a link to the specific file you might want to look into for guidance.

https://github.com/wojtekmach/acme_bank/blob/master/apps/master_proxy/lib/master_proxy/plug.ex