I have a Netgear WNDR3700 v2 (ar71xx). Trying to update to the latest version of openwrt using sysupgrade via luci, I get an error message saying "image not supported".
As far as I remember, there should be nothing special, although 12.09 is so ancient that I do not remember if there was some ID change in 2012-2015 before 15.05 (or late before 17.01 and 18.06).
But the current images work ok in 3700v2, so there should be nothing special in the update itself. (I test my WNDR3700 series community build on 3700v2)
You could
download the image with wget to /tmp on your router
use "sysupgrade -F -n" to flash it from ssh console. Hmm... I just wonder if the -F option was present already in 12.09 or has bit been added later...
Alternatively, you might try upgrading first to 15.05.1 and then to 18.06.1
The sha256 checksum b3ca0f3483a897913df38275b59baaade2ddc4a8918579c47de81209b2a47f57 matches but the checksum luci gives before proceeding to flash is e81892028e37d9440bbeaa9ee8bf9acf . How can I verify this?