I am not exactly sure, what you have currently, but this is my suggestions for upgrading to 6.51.
on the open at 3.02 cd there are the new core firmware in wdwl 1k-xmodem format. I saw strange things with at+wdwl (if the ‘$’ characters don’t start instantly, a download can end with a dead modem so start again with eprom files)
A better method is the dwlwin procedure, try to obtain the new files for it.
You need a special cable to upgrade the fastreck modem to pull down the boot pin.
In 15-pin Fastrack interface:
BOOT: pin 3
GND: pin 9
The BOOT signal is active low so just connect BOOT pin to GND via a 1k ohm resistor during power on reset and you’ll be able to use the DwlWin application.