Try to reset the BIOS, see informations below,

See : http://www.ourdreambox.com/ubbthreads.php?ubb=showflat&Number=3259#Post3259

Generally the IP address is set by the MAC address, that is contained inside the Sim card.

Try to see if your MAC address ("ifconfig" command in ssh/telnet when you are connected on console mode in the dreambox) is not set in the router to a fixed IP address.

The other thing, could be that by software the IP is fixed (inside the image or perhaps the BIOS ?).

Admin