Suggested reading:
https://openwrt.org/toh/tp-link/tl-wr841nd#go_back_to_original_firmware
https://openwrt.org/toh/tp-link/tl-wr841nd#tftp_installrecovery_via_serial
https://openwrt.org/toh/tp-link/tl-wr841nd#tftp_recovery_via_bootloader_for_v8_v9_v10_v11_v12