I presume you are using port sharing to share access to the ports. If so, you will want to apply the exclusive control mode to have the setting exclusively controlled from the Digi unit:
set sharing control=exclusive port=#
From the Command Reference Guide:
exclusive:
The serial port parameters can only be set from the command line or
web UI of the device itself. Any attempt by the clients to change serialport
parameters through RealPort will be silently ignored.