The IP address (for any device) would need to be reserved in the DHCP IP lease pool.
If you rather not have to deal with reserving static IPs, you can always craft firewall policies based on the device mac address. To do this, first enable Device Detection under the interface (e.g. LAN) then go under "User and Device->Device Inventory", locate the mac address (if you do not see the MAC column just right-click on the column headers and add it), and edit that mac - give the device a name and create or add that device to a new device group (e.g. unfiltered-devices). When it comes to crafting the firewall policy - much like before you would use the source "all" followed by the device group name. The rest of the firewall policy should remain the same.
