Does this work with barebones Elixir / no Phoenix?

I'm trying to use this in a project without Phoenix and, with the same exact credentials, it will work on the Phoenix project but not on the Elixir one. It keeps throwing the "invalid_client" and "The OAuth client was not found." error after I request the token. I am running the server and client separately.

Here's the code:

post "/" do
  conn = Plug.Conn.fetch_session(conn)

  code = Map.get(conn.body_params, "code", "")

  {:ok, token} =
    ElixirAuthGoogle.get_token(
      code,
      "#{Application.get_env(:root, :frontend_url)}"
    )
    |> IO.inspect()

  {:ok, profile} = ElixirAuthGoogle.get_user_profile(token.access_token) |> IO.inspect()

  ...
end

Am I doing something wrong? This is the only relevant code to the project, and it is throwing after ElixirAuthGoogle.get_token/2