Problems with setting linksys wrt54g as access point

My setup is this:
office at one end of house has direct cat5 line to other end of house.
In office I have a Toshiba modem connected to a linksys wrt54gs
router.

I tried to set a second linksys router (wrt54g) as access point to be
connected to line at other end of the house.

I connected the second router to a single computer, set a new base
address (192.168.1.156), turned off DHCP, and made sure the security
setting was the same as the first router. As soon as I saved the
changes, my primary router was disabled, forcing me to reset it and
set it up again. Have tried this twice and assume I am overlooking
something basic.

Any advice?