Okay... The wireless router is behind the wired one, right?
Can a machine connected directly to the wired router get an IP?
Can a machine talking to the wireless router (with a static IP) ping the wired router?
When you tried to "extend" the network, was this problem already existing, or did it occur as a result of the "extension" that was made?
Take a look at the wireless router's web interface. Check the DHCP settings and be sure they are correct. Also, see if you can get it to display its routing tables, see if they look correct.
~psi42