We used mostly RB411U because of their RS232 port for communication with external device on long distance.
Special-login and remote port commands are used and that work ok.

Now I have request to use same RB411U with 3G modem installed but some "dummy" device have only RS232 and on special trigger event must send AT command to 3G modem which is installed on RB411U.
Dummy device and RB411U are connected with serial cable only !
/system serial-terminal usb2 channel=x is what I need but from RS232 instead terminal.

Anybody have some hint ?