The V2 looks to be very similar to the V1, but neither is a good router to run OpenWrt due to the lack of an open driver for the wifi chips. It would only be useful in a wired role. That is likely why there hasn't been development for the V2.
If you want to run third party firmware on your Archer C9, checkout the posting for the latest beta release in the dd-wrt forum, Broadcom Devices sub-forum, for any serious bug reports first, then flash the latest beta. I ran dd-wrt on an Archer C9 V1 not long ago and it was good stuff. Like mk24 said, OpenWrt just isn't made for a Broadcom device like your Archer C9. Sending you over to dd-wrt is the best advice, or keep it on stock