Help flashing Asus RT-AC68U

I have two routers that I would like to flash with OpenWrt. One is a Asus rt-ac68u and the other is a Netgear R7000. Both are running the latest Merlin firmware. I flashed the Asus back to stock and then tried OpenWrt but got the message saying the firmware was a 3rd party and would not install.
Does anyone know a way around this?

Are you aware that the support for the WiFi chip (BCM4360) in your RT-AC68U is very limited on OpenWRT?
So, if you manage to flash OpenWRT on your router, you will only be able connect at 802.11g (WiFi g) speeds, which is 54 Mbps max.

I've updated the RT-AC68U wiki with some warnings borrowed from the Netgear R6250 wiki, which is a similar device. Also, I believe the Netgear R7000 is similar too.

Yes I found that out when I did get it flashed. Reverting back ddwrt now. thanks

newbie, i to trying to flash rt68u go the same message as in your first post , not bothered about wi-fi just using wired, can you please post up how you eventually got open wrt flashed ont the 68u

update got open wrt working, i remember adding advance tomato so i used the same ceo for that putting the open wrt fw , now to start checking things out