I had some difficulties installing OpenWRT 21.02.1 on my TL-WR840N Brazilian version (BR) and now I'll share with you my success path.
I installed the latest OEM European (EU) firmware version (
TL-WR840Nv4_EU_0.9.1_4.16_up_boot[211109-rel73480].bin). It kind of brick my router and a I had to enter in recovery mode. Thanks it has a quite easy recovery mode!!!
For any reason I just don't know I hadn't success with the factory OpenWRT firmware (
openwrt-21.02.1-ramips-mt76x8-tplink_tl-wr840n-v4-squashfs-sysupgrade.bin), so I tried the upgrade OpenWRT firmware (
openwrt-21.02.1-ramips-mt76x8-tplink_tl-wr840n-v4-squashfs-tftp-recovery.bin), and it worked.