Try other baud rates, especially 115200. Some boards do not reset and hence not send "start" on connect, which is ok. But they should respond on commands send which does not happen here so a wrong baud rate is probably the issue. Also make sure you have main power enabled as well. Some boards show port while not having firmware running like the Rambo einsy which you may have. In that case connection also only works when board gets main power form printer PSU.