Setting Source Address MY

Hi, I have been trying to set the source address of an XTend-pkg rs232 modem to FFFF. I have tried with the config manager and AT commands, and then write this to the modem. It is ok until I power down. After powering up it reverts to the original source ID.
This is to make it easier to use different remote modems with our local unit.

Any Ideas???

Yes, set he radio for user defined mode which is dip switches 5 and 6 both in the up position.