Common causes of getting 100Mb/s connection rather than 100Mb/s are faulty Ethernet cabling or perhaps negotiation/ speed settings between the Fortigate and the modem/ internet device. SMB Protection (IPS, Advanced Malware Protection, Application Control, Web & Video Filtering, Antispam, plus FortiGate Cloud subscription and FortiCare Premium) FGT_30E_MN-v6-buildxxxx-FORTINET.out – North American build – supports frequencies used by carriers in North America only. Serial number starts with: FGT30EN

Maximum firewall throughput is 950Mb/s and if you use full threat protection (which you should) maximum throughput is about 150Mb/s (depending on traffic type and mix). One of the really cool features of the FortiGate 30E is the Traffic Shaper. In essence, you can assign a high priority to VoIP traffic and a minimum amount of bandwidth to ensure you clients can hear you clearly and easily. In most units, this is done either by a Serial cable or an RJ-45 to Serial cable. There are some units that use a USB cable and FortiExplorer to connect to the console port. Ideally, you probably do want the D-Link to act as a pure modem- but you would need to reconfigure it (if that is even possible?) and let the Fortigate act as the only router on your network. The Fortigate may then need to run PPPoE (for example) depending on how the ISP manages connections.Lastly, be aware that the 30E will not support full 1Gb/s (1000Mb/s) throughput. Have a look at the datasheet here:- High performance UTM - firewall, VPN, IPS, application control, web filtering, antivirus, antispam, DLP and more

From your description it sounds like the D-Link "modem" is actually acting as a router. If you can connect a PC directly to the "modem" then it sounds like it is running DHCP (and assigning the client an IP and DNS settings) and acting as a NAT router. Once logged into the FortiGate with the maintainer account (as described below), if the FortiGate is running FortiOS 6.0.3 or later, enter the execute factoryreset command to return the FortiGate to its default configuration.https://docs.fortinet.com/document/fortigate/7.2.4/fortios-release-notes/230510/changes-in-default-b...

But, there is certainly nothing (even with the 30E) that will prevent you getting that speed reliably if you have it correctly configured. It will be a config issue and certainly not a bug or Fortinet defect. These basic configs work well on Fortigates and are well validated and tested. But it may take time to work through it. Also you need to look at the DNS server settings on the Fortigate (the Fortigate defaults to the Fortinet DNS servers). You may find you getting better/ faster name resolution using your ISPs servers and then just using the Fortigate for SDNS filtering. FortiGate-30E is a special unit available in several hardware configurations; FortiGate (no wireless modules at all), FortiWifi (wireless a/b/g/n). Unified Threat Protection (UTP) (IPS, Advanced Malware Protection, Application Control, Web & Video Filtering, Antispam Service, and 24x7 FortiCare) Your "diagnose hardware deviceinfo nic wan" shows that too- the "Speed 100" agrees with what the AMBER speed LED indication is showing you.So, if you have a 1Gb/s (1000Mb/s) internet connection (for example) you wont get more than 100Mb/s speed until the WAN link is also showing a "GREEN" Speed LED and your diag output shows "Speed 1000". Designed for small environments, you can simply place the FortiGate/FortiWiFi 50/51E on a desktop. It is small, lightweight yet highly reliable with superior MTBF (Mean Time Between Failure), minimizing the chance of a network disruption