Double-check the sticker on the bottom of your router. If it says V6.0 or V6.1, the V6.20 firmware might be rejected by the system.
The "ME" in TL-WR840NME is a crucial character in this story. It stands for the Middle East and Africa market. tlwr840nme v620 firmware full
If a "full firmware" flash went wrong, or if the Power LED is blinking rapidly/non-stop, you can use the : Double-check the sticker on the bottom of your router
If your router is a specific “ME” version and not on the global site, try: tlwr840nme v620 firmware full