Hi Marcin,
So in that case, answering your question, yes you can translate ethernet protocol into a rs232 port, but in that case you will need a TRB142 or a RUT955. In that case, you would connect, for example, a RUT955 to your GSM Module using the RS232, and your SCADA software would send the AT commands over internet to the RUT955 IP address, and the RUT955 would send those AT commands to the GSM Module, using RS232, and vice versa, thus completing the communication, and you don't need the computer and the rut955+GSM module to be in the same room.
But, if you already have a TRB140, i'd recommend you in your case, to use its SMS gateway functionality, that way you can replace your GSM Module and reduce complexity.
Best regards.