Replace it under warranty.

Some firmwares are available here. Did you try? http://www.tp-link.com.my/download/TL-WDR4300_V1.html#Firmware

Actually, we cannot SSH into the router. The firmware can support SSH because of Tether 2.0 which is an APP used to manage and configure routers. To guarantee the safety of the communication process,...

Where did you download the firmware 150902? I cannot find it on TP-LINK website. You tried to upload the firmware Archer_C2_V1_141119(Unifi_and_Maxis_Fiber) here but failed?...

Firmware is up to date? What's the model number of wireless card on the client connecting to 5GHz Wi-Fi?

What's the model number of wireless card? You tried on different wireless clients? How about signal strength? Try channel width 40MHz.