Ok, last update before I call it a night:
1. why 5 routers: I don't need 5 routers per se, but I do want 5 Mesh access points to maximize the bandwidth across my 3-story brick and concrete house. In my living room (where I have the Zenwifi XT8 (AX6600) ROUTER), I get 600 Mbps. If I only use the second XT8 on the top floor (home office), that drops down to less than 200. Adding Mesh points should reduce the drop in speed.
2. I just did the full reset on the ac68U (power off, connect the WAN to the Zenwifi, connect a LAN port to my PC, hold thw WPS button, power up, wait for the led to start flashing, let go and wait for the boot up to complete). Nothing has changed. I can browse to the ac68U and set it to Mesh node and then nothing. It will not find it. Either I am too stupid for this stuff (which is possible), or it just doesn't work. But then, why does it work fine with the other ac68u already in the mesh?
3. When I browse to my Zenwifi (either from a PC in a browser or from the app on my ipad or iphone), it keeps telling me firmware upgrades are available (and clearly important as the setup keeps telling me to make sure the latest firmware is installed). But when I try to do the upgrade, it always fails. On my Ipad, it goes through a 7 minute procedure and then tells me it failed, try again. On the PC it just says 'downloading firmware' with no progress or next steps, even after a few hours.
So I am stuck in this cycle where I can't upgrade and I can't add nodes without upgrading (presumably).
Why doesn't Asus just make the forware downloadable to a USB stick and we can just flash each router/node manually? Too old school?