RS232 cables can be a nightmare as all the handshaking (RTS/DTR/CTS/DSR) that may or may not be needed or is needed by one side of the connection but not supported on the other side. On top of this the type of equipment your connecting to may need the RX/TX wire crossed or uncrossed.
I would suggest you start by confirming the cable is wired okay. As hardware handshaking is not needed by default (I assume your equipment is default) the minimum connections between the DB9 & DB25 connector need to be either
DB9 pin 2 - DB25 pin 3
DB9 pin 3 - DB25 pin 2
DB9 pin 5 - DB25 pin 7
or
DB9 pin 2 - DB25 pin 2
DB9 pin 3 - DB25 pin 3
DB9 pin 5 - DB25 pin 7
Start with just sending the N command as it should return a software version value. If your not getting a reply then try again with the alternate serial cable connection.
If this still fails then check your Serial shield by looping a wire between DB9 pins 2 & 3 and anything you send should be echoed back to your sketch. If this fails then it could be a dodgy sketch or RS232 shield hardware.