I think @Fady pointed out the most likely solution to your problem. In many cases you can 1:1 NAT. Go to Security Appliance>Firewall and in the 1:1 NAT section just put the public IP and point it to the LAN IP of the device you want to have that IP and you'll likely be good to go.
Adam R MS | CISSP, CISM, VCP, MCITP, CCNP, ITILv3, CMNO
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