Can you use ssh to go into the device?
If you do that, you can enter:
set ipaddr wan dynamic
and
reboot
After that you will lose connection to the device, and it will reboot and ask your dhcp server for an address. Check the logs of the dhcp server to see what address it got.
Of course, this could also be done using the web ui, but this way was easier to explain.
Hope it helps.