Issue resolved, the problem was cause by putting a space character in SSID. Finally have a full speed download rate after removing it. But still don't understand why a space character would effect...

Model : TL-WDR3500 Hardware Version : V1 Firmware Version : 150518 ISP : HGC Broadband Hong Kong I understand it sounds weird but it just happened to my TL-WDR3500... by using the default...