Full disclosure, I have never configured this before. But reading through the docs it appears as though the LAN behind the FEX shares the same subnet as the LAN Extension interface on the FGT. So if you have an existing LAN using 192.168.1.0/24 on the FGT, you'll probably need to create a new subnet 192.168.2.0/24 for the LAN Extension interface. Then, you can use this subnet on the FEX LAN as well and point clients to the FGT as the default gateway.
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.