Could you tell me what is my problem here ?
Must be the blue shirt you are wearing. Try a green one.
See, that answer makes as little sense as your question. Not that there is a problem with the question, but there is absolutely no context for it, and it is completely unrelated to how to use a 5 meter cable to stretch 10 meters.
What operating system are you using? Clearly, it isn't windows.
What application are you trying to use to talk to the serial port?