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.

Installing firmware intended for TL-WR840N v5 or v6.0 onto your v6.20 will brick the router (turn it into a blinking paperweight). The "v620" specifically requires a bootloader that matches the flash chip layout of that hardware run.

A small but significant feature in this firmware iteration was the improvement of the bridging function. Users in large homes would often buy two WR840Ns to link them wirelessly and cover a dead zone. Previous firmware versions were finicky with this; 6.20 smoothed out the handshake protocol, making the bridge more resilient to signal drops.