Flashing different firmware is risky, even for the same variant but switching regions... More so when switching variants.
Changing the Firmware will not give you a radio frequency that is physically absent from the hardware.
The RM-821 is a Euro style radio equipped device. It is doubtful the 1700 AWS radio is onboard and just hidden. The RM-820 would have a better chance of 3G 1700 being hidden.
Before you do anything drastic, see if you can access Field Menu by dialing ##3282#. If you can, follow the example of how to enable 1700 AWS that is given here:
http://forums.windowscentral.com/nok...-4-1700-a.html