Now, there are tutorials for getting DD-WRT firmware on the R6900 and R7000 routers. But nothing about running the XWrt-Vortex firmware for the R7000 on the R6900. So I figured I'd give it a try based on the available information. And the following procedure worked for me:
Step 1: Reset Netgear firmware to default.
Step 2: Acquire TelnetEnable specific to the Netgear router (R6900/R7000). I use Linux so I did
>git clone https://github.com/insanid/NetgearTelnetEnable.git
>gcc -o telnetenable md5.c blowfish.c telnetenable.c
>chmod a+x telnetenable
The mac address can be found at the bottom of the router. user name is admin password is password. IP is probably 192.168.1.1, so something like ./telnetenable 192.168.1.1 F541052345 admin password would be run.
For Windows you'd find the windows version specific to Netgear routers and do essentially the same thing.
Step 3: Once telnet is enabled through that exploit, run telnet to the same ip you just used. Then once you're in, you can run the following commands:
which makes your router identify itself as an R7000
>nvram erase && reboot
which erases your nvram and reboots the router
Now wait for your router to reboot before proceeding with Step 4.
Step 4: Download and install the R7000 XWrt-Vortex firmware from the following website...
Unzip it and then log in to your netgear router (again, probably 192.168.1.1)
Go to Advanced settings and update firmware. select the firmware you just downloaded in my case at the time of writing it was R7000_380.67_0.chk, then click on update. Wait for the router to update and reboot.
Step 5: Enter in to your router and voila, XWrt-Vortex. I had saved my settings from my old Asus-WRT Merlin router and I was able to update all those settings to the newly installed "R7000" router. Simply uploaded my Settings_RT-AC66U.CFG file and the new router was more or less configured the same as my old Asus router.
And voila, finished. Seems to be running quite well.
You could probably Follow Steps 1-3 and then install Tomato or DD-Wrt for the R7000 instead. Maybe even OpenWRT although at the time I write this there doesn't appear to be support for the wireless broadcom chipset in OpenWRT. But in any case I haven't tried any of those options as I'm quite happy with AsusWRT/XVortex
Also note, there is a version 2 of this router, this process will only work on the version 1. Or as its labelled on my box barcode "R6900-100NAS"