The URLs for “Firmware OpenWrt Install” and “Firmware OpenWrt Upgrade” are reversed; for example, the tftp-recovery file is on “Firmware OpenWrt Upgrade” when it should be on “Firmware OpenWrt Install”, and the sysupgrade file is on “Firmware OpenWrt Install” when it should be on “Firmware OpenWrt Upgrade”.
https://openwrt.org/toh/tp-link/tl-wr840n_v4