The speedtest App is based on your neighborhood server to test and estimate the speed from your WAN to the selected server. Selected server could be the limiting factor during the speedtest. if you are sure that your ISP can support 1Gbps, just go ahead and modify the download and upload speed to 1000Mbps, the APP accept manual setting. After that you may turn ON FasTrack for router QOS to take effect. Hope this help.