laptop in ip fixe , cut wifi , open firewall
install and open tftpd
rename firmware tp-firmware.bin
reboot router will press reset
WR840N firmware by tftpd
WR840N v6 firmware openwrt
https://openwrt.org/toh/hwdata/tp-link/tp-link_tl-wr840n_v6
Firmware version:0.9.1 4.19 v0001.0 Build 220120 Rel.55489n
Hardware version:TL-WR840N v6 0000000https://firmware-selector.openwrt.org/?version=19.07.4&target=ramips%2Fmt76x8&id=tl-wr840n-v4
https://github.com/IcedShake/openwrt-19.07-tl-wr840n-v6.x
Compiled file for OpenWrt V6