So first off, love the router as it replaced a dying ASUS AC3200. Very easy to set up, very fast throughput!
However in my home I have some advanced things going on that lead me to not use the routers DHCP server and I can't change the network address range without significant pain and suffering on my part.
The AX11000 kept changing from the default 192.168.0.1 to 192.168.1.1 which would of course break everything. I suspect because it detected an issue connecting with my ARRIS S33 which uses 192.168.100.1. (The ASUS didn't allow me to navigate to the modems status page but it never complained about the 0.1 IP address I assigned it!)
Anyway, the solution seems wonky to me but it makes sense. I'm sure some networking guys would shake their head because it seems like a cheesy way out but whatever, it works.
Just go set the routers LAN IP back to 192.168.0.1 and set the subnet to 255.255.0.0. This tells it all networks in the 192.168.*.* range are valid. It created a proper route in the routing table and everything works perfectly.
I found nothing on the web to help so I'm posting this in hopes that it's indexed and if anyone else has the issue, because of the reasons I had them (cannot change their network range) this may help them too!