Following is an example of Fortigate static NAT configuration, to statically translate the internal IP address 10.1.1.10 to 188.8.131.52
config firewall vip
set extip 184.108.40.206
set extintf "port1"
set mappedip 10.1.1.10
Following the VIP configuration, be sure to configure the policies to allow flow of traffic.
The policy that allows traffic coming from outside (Internet) should reference the VIP IP.
The policy that allows traffic from inside (Trusted or DMZ zone) should reference the original IP.