Hi Lukas,
Regarding any button to send to RS232 via RMS, I believe there is non for this RS232 functionality is having the different modes (Console, Modem, Over IP, and Modbus Gateway) it will vary on the configuration you are using. So it is best to finalize things from your side what are you trying to do rather than giving broad answers. There should be a TCP/IP port to which the RS232 device is listening in order for you to reach the connected device Over IP.
If your TRB142 device is connected to RMS successfully it should show offline as default there is a 1-month free subscription for RMS when the device finished the one-month duration you need to buy RMS credit to make it active again.
Regards,
Mellow