Many people could not compile alone so i decided to upload some bin builds.
I included only luci and IPV6 is DISABLED. It could be included too but i need more time for this. Here are the links:
TP-Link 840 V6.0 - http://212.50.69.96/mt7628/openwrt-ramips-mt76x8-tplink_tl-wr840n-v6-squashfs-sysupgrade.bin
TP-Link 840 V6.1 - http://212.50.69.96/mt7628/openwrt-ramips-mt76x8-tplink_tl-wr840n-v6.1-squashfs-sysupgrade.bin
TP-Link 840 V6.2 - http://212.50.69.96/mt7628/openwrt-ramips-mt76x8-tplink_tl-wr840n-v6.2-squashfs-sysupgrade.bin
TP-Link 841 V14 - http://212.50.69.96/mt7628/openwrt-ramips-mt76x8-tplink_tl-wr841n-PWR-v14-squashfs-sysupgrade.bin
I think i can build for any mediatek 7628.
You can upload directly from original software.
For returning back to original use TFTP.