TL-WR840N v2 is not listed on the Suppport Device List: http://www.dd-wrt.com/wiki/index.php/Supported_Devices so apparently it is not supported by dd-wrt. It is not wise to flash a TL-WR840N v1 firmware to a TL-WR840N v2 as they may have different hardware. There are even 2 different versions of v1, one is the China Only v1 version which use Atheros CPU and is supported by dd-wrt and the other one is the non-China only v1 version which use Broadcom CPU and it is not supported by dd-wrt as far as I know.
_________________ Before asking a question on the forums, update dd-wrt: Where do I download firmware? I suggest reading it all.
Some dd-wrt wiki pages are up to date, others are not. PM me if you find an old one, I am trying to update them.
Atheros:
Netgear R7800 - gateway, DHCP, QoS, VPN
TP-Link Archer C7 v2 x2 - WDS AP, WDS Station
TP-Link TL-WDR3600 v1 - WDS Station
TP-Link 841nd v8 - WDS Station
D-Link 615 C1 x 4 - not used
D-Link 615 E3 x 2 - not used
D-Link 825 B1 - WDS Station
D-Link 862L A1 x2 - WDS Station
Netgear WNDR3700v2 - WDS Station
UBNT loco M2 x2 - airOS
Broadcom
Asus N66U - backup Gateway
Netgear r6300 v1 - AP
Linksys E2500 - not used
Linksys EA2700 - not used
Linksys 160N v3 x2 - not used
Netgear WNDR3700v3 - not used
MediaTek
UBNT EdgeRouter X - not used