Wireless connection is not as fast as wired. As for WDR3600, the maximum wireless speed could only reach about 300Mbps no matter on the 2.4GHz or 5GHz band.
Even the wireless speed displayed on the status page could reach 300Mbps, it's actually not the real speed displayed when you're downloading. 300Mbps is the speed on the physical layer, when you're downloading, some necessary headers of packets will also take up some bandwidth, causing the actual download speed to be only half of the physical speed or lower due to some interference in the environment.
As for your situation, it's recommended to upgrade a higher-level wireless router which has higher wireless speed.
You could learn something about the WLAN protocols(802.11n&802.11ac) on the tplink's official website. It's just useful for me.

Note: Wireless speed will also be limited by the client devices.If the clients don't support the 802.11ac protocol, its wireless speed will also have a bottleneck even you upgrade a high-end router.