The PC's RS232 port is male DB9
The PLC's RS232 port is female DB9
So if your cable (I assume is female DB9) works with the PC, then you need a male-to-male cross over cable to connect that to the PLC with the following pins:
2 - 3
3 - 2
5 - 5
The PLC actually only needs 2,3 and 5 and doesn't need the other signals but if your modem need those signal in order to work then you will still need to short them such that 7,8 are shorted, and also 4, 6 are shorted.
After that, try to verify if the PLC can activate the GSM modem to do something simple, such as dial a phone number. If it can, then proceed to the next step.
Note that in order to program in TBASIC to send the quotation character (") to the modem, you need to use the CHR$(34) to construct the string:
To send: AT+CMGS="#_mobil"[Enter]
PRINT #1, "AT+CMGS="+CHR$(34)+"#_mobil"+CHR$(34)