FortiGate is operating in NAT mode and has two physical interfaces connected to the LAN...

Both interfaces must have directly connected routes on the routing table

In NAT mode, each interface must have a corresponding entry in the routing table, typically as a directly connected route, to route traffic between them effectively.

Both interfaces must have IP addresses assigned

In NAT mode, each interface must have an IP address to participate in routing and NAT operations. The IP addresses allow the FortiGate to forward traffic between different network segments.