Rerun command if it die in elixir

Hi, I use m3u8 to live stream audio, I use ffmpeg to convert from webrtc to m3u8. but some reason ffm (port not avaiable) can not start or it die when running. I want to Rerun it again with other port.
I use Supervisor and GenServer to do it. here my code:

defmodule FFmpeg.Supervisor do
  use Supervisor

  def start_link do
    Supervisor.start_link(__MODULE__, [])
  end

  def init(_) do
    children = [
      worker(FFmpeg.Service, [])
    ]

    supervise(children, [strategy: :one_for_one])
  end
end

defmodule FFmpeg.Service do
  use GenServer

  def init(:ok) do
    {:ok, :ok}
  end

  def start_link do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end

  def start_ffmpeg(room) do
    GenServer.call(__MODULE__, {:check, room})
  end

  def handle_call({:check, room}, _from, state) do
    exec = "ffmpeg -analyzeduration 300M -probesize 300M -i #{sdp_file} -strict -2 -c:a aac -ar 16k -ac 1 -preset ultrafast -tune zerolatency  -f flv #{path_rtmp_janus}/stream_#{room.id} -nostdin -nostats >/dev/null 2>&1 &"
    Port.open({:spawn, exec}, [:binary])  # how to RERUN if it die
    IO.inspect "========= END run ffmpeg =============="

    {:reply, :ok}
  end
end

and run by:

if !Process.whereis(FFmpeg.Service) do
        FFmpeg.Supervisor.start_link()
end

 FFmpeg.Service.start_ffmpeg(room)