To debug DHCP server feature, you would eventually need Wireshark or something to sniff dhcp handshakes between clients and the server(FG90D) on LAN. I'm guessing either FG90D doesn't like what clients are sending or the requests might not be reaching to the FG. But first I would sniff the packets in and out on the interface at the FG (diag sniffer packet). I think you can use 'dhcp' as protocol to capture only what you want to see.
For the version you are running, v5-build292 is 5.0.9. Since that version it's gone through 5.2.x and 5.4.x major versions and now 5.6.0 is available. I would recommend upgrading it up to 5.4.x at some point.