I have FortiGate 300D that has 2 management ports, and I have to use this port like other LAN port. But it cannot route to other ports, or even access internet. I'm sure with my policy. Is there any additional configuration in order to use this management port?
It's like Ede said, they cant be used for traffic forwarding. If you need more interfaces look at the occupied ones and see if interfaces can be freed up, such as making trunks with VLAN interfaces instead of single interfaces. I see your device, FG300D, has 4x 1GB UTP and 4x 1GB SFP. Most likely all UTP ports are used but not the fiber ones? There are tranceiver SFP's available which make your SFP port UTP ports.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.