Feel free to try. Mine's currently running for 2 days now with SQM turned on, with about 4mb still free on ram.
sysupgrade: https://www.dropbox.com/s/185otrdv7rjph02/openwrt-ramips-mt76x8-tplink_tl-wr840n-v6.2-squashfs-sysupgrade.bin?dl=1
tftp-recovery: https://www.dropbox.com/s/dot2666znxltb1n/openwrt-ramips-mt76x8-tplink_tl-wr840n-v6.2-squashfs-tftp-recovery.bin?dl=1