We have an 8840MFC. Not sure if your's is the same, but you can determine the printers default IP address ( or even set ) using the little LCD screen and one of its menus. I'm assuming that it is networked via ethernet, BTW.
Sometimes, it is easier to simply Delete a printer in the Printer Setup panel and then Add it again. Of course, that assumes you can see it...
I think you may also have some Brother software, Brother Control Center? You can tell it how the printer will be connected; USB or Network. That may also help.
Finally, try turning it OFF/ON while the Mac(s) are shut down. Then restart them.
BTW, even if your ISP requires DHCP, you can assign IP addresses to everything on your LAN ( even if that's just the printer and the Mac ). This will prevent getting different addresses every time something gets turned OFF, especially if you have more than one computer on the LAN.