XBEE PRO SX : Bootloader not invoked by recommended method using Serial break ON, Reset and Serial Break OFF

Hi ,
I am using XBEE PRO SX : 900DM, i am not able to invoke bootloader using USB serial port. I have already tried DTR=checked, SerialBreak=checked, then apply hardware Reset , the Serial break=unchecked, then press B and CR or CR. The baudsrate setting: 115200bpw,1N1

Please what to do , since i need to update the secondary firmware into that.