Re: 500 gig symetrical bandwidth increase
The duplex/speed of the devices at both sides of that WAN connection should be left at auto negotiation. No actual need to adjust the duplex/speed on the fgt side unless you see any rx/tx error counters increasing on the port (via diag hardware deviceinfo nic <interface name>).
For a 500 Mbps symmetrical line connection, the WAN port connection to the WAN device should be at least showing 1000M full-duplex (if I am not mistaken).
Traffic shaping is generally not used/applied to the WAN connection - you may be thinking of rate-limiting. (Traffic shaping is mainly applied via firewall policy rules).
On the GUI, check the WAN <interface> connection and see if the Estimated Bandwidth values are set - if not, you should set them according to the expected bandwidth.
Keep in mind that applying various UTM features (via firewall polices) will slow down your device connection (behind the firewall) based on the traffic and amount of packet inspection involved.
When it comes down to it, you need to first confirm/determine if there is a connection/speed or other factors involve and the best way to do that is direct connect your laptop directly to the ISP equipment and run speeds tests that way.
NSE4/FMG-VM64/FortiAnalyzer-VM/5.4/6.0 (FWF40C/FW92D/FGT200D/FGT101E)/ FAP220B/221C