Serial Modem - Which cable to use?

Hi to all,

I’m trying to connect a Us robotics 995630 56kbps modem to 450g.
Although the modem is not using hardware flow control , I get no response in my commands in serial terminal.
I verified that

  • port is not used by console
  • Modem is working (from windows with port settings 115200,8,N,1, none flow control)
  • Mikrotik is able to autodetect baud rate (115200) from serial terminal

The cable I use is a normal (not null) Rs232 25-9 .
Should I try to use null cable? If yes, which type of null cable (with loopback or not)

Thank you very much in advance,