PORT configuration not saved after next power cycle

in UBX-CFG-PRT, I configure USB port to output NMEA messages, it does start giving NMEA messages but when the simple RTK2B is connected through USB port
again after disconnecting it, It does not output anything & I have to configure the USB port again to output NMEA.This happens even after I have saved the changed configuratuin in UBX-CFG-CFG window.Please suggest the solution.