LEDE network havoc

I'm new to LEDE, having installed it onto a Buffalo WZR-HP-AG300H since a few days.
My primary router facing the internet is a Linksys E3000 running Tomato reliably for several years.
This gateway router is connected to 9 wired WiFi access points serving 30 users who all have static ip's.
I inserted the Buffalo/LEDE router as the 2nd router after the gateway router connected via LAN, and configured it as a dumb access point/switch via the LuCi interface. Since then, My network has gone bezerk. 1) WiFi dropping in and out 2) WiFi giving incorrect password errors 3) Clients unable to join the network 4) Internet inaccessible 5) Individual routers GUI being inaccessible from the LAN.

If I remove the LEDE router from the network, everything settles down.
I will try to reset the router and reconfigure as a dumb access point using ssh.
If anyone has had similar issues, I'd appreciate your assistance.

Semirandom things that come to mind:

  • Is the Buffalo, even in "dumb AP" configuration, running a DHCP server?
  • Are your existing 9 APs in a roaming configuration, and are you trying to add the Buffalo to that mix? Or does the Buffalo have its own WLAN SSID? If you're trying for "roaming", are the semantics of the WPA key (I'm assuming you're using just PSK here) the same on Tomato vs. OpenWRT? ISTR that in PSK the WPA passphrase is hashed with the SSID to yield the underlying shared key, could it be that Tomato perhaps isn't doing that? WPA key calculation

Yes, make sure your DHCP server is turned off, both IPv4 and v6. The check box on the first page only disables IPv4. You have to click the IPv6 tab and disable that separately.