Frequent IP address conflicts on RAX80 net, even with defined device ip addresses???

> I have RAX80 router [...]

   Firmware version?  Configured as a router?  Connected how, exactly?

> [...] behind AT&T ADSL router. [...]

   Maker?  Model number?  Which way is "behind"?  What, exactly, is
connected to what, exactly?  (Hint: If a device has different types of
Ethernet ports, then "connected to device" is not enough detail.)

> [...] quite frequent problem with assigning same IP address to
> different devices. [...]

   Netgear router firmware may be buggy, but the usual explanation for
that sort of behavior is having multiple DHCP servers on one LAN
segment.

> How can I fix this and have stable IP addresses for all my devices?

   If your (unspecified) "AT&T ADSL router" is configured as a
modem+router, then you might want to configure the RAX80 as a wireless
access point.  Visit http://netgear.com/support , put in your model
number, and look for Documentation.  Get the User Manual.  Read.  Look
for "Set up the router as a WiFi access point".