Fortigate 300C network problem

Hello!

I have two Fortigate 300C devices in two buildings. The connection is a layer 3 connection with static routes, as shown in the uploaded picture. The connection between the two Fortigates is configured on port 2. Also on Port 2 there is a tunnel interface with an IP address. The routing configuration between the Fortigates is static routing. On port 1 on both Fortigates is the connection to the local network of both buildings (connection to the main switches of the buildings).

Now I want to transport a second network from the Fortigate 1 over Port 2 to the Fortigate 2 device. This second network is connected on port 3 on the device Fortigate 1. The port on the Switch pointing to the Fortigate 1 is tagged, with one VLAN (VLAN ID 20). I want to transport this VLAN 20 tagged from port 3 (Fortigate 1) over the Port 2 to the other device Fortigate 2 and connect a switch on Port 3 on Fortigate 2. This switch will add this tagged VLAN 20 on his uplink port and then the other Ports will be untagged.

How can I configure the Fortigates, that the Layer 2 network will be transported over the Layer 3 network? I've tried to create a vlan subinterface on port 2 and then created a software switch, combing the vlan and the port 3, but it sadly didn't work.

I look forward to your reactions.

Mario