I have found, after setting up mixed wired/wireless routers to cable modem/dsl that it takes a few resets/reboots of all connected devices before everything *just* works.
Specifically the order I usually set up the router is as follows:
Get your DSL account info if you need to re-enter it after resetting the cable/DSL modem.
Plug in an Ethernet cable from a computer to the router.
Power on the router.
Load the admin web page in a web browser (Linksys is
http://192.168.1.1, D-Link uses
http://192.168.0.1). Linksys doesn't have a user name, just a password while the D-Link has "admin" as the username and no password (just press enter) - go figure.
Then I go into the Admin screens (or setup) for the LAN via the router and make sure it looks okay. I always turn off remote admin/wireless admin.
Next CHANGE the admin password on the router!!! Make sure you right it down as you may need to re-enter it several more times.
UNPLUG the cable/DSL modem from power. Typically the modems attach themselves to a particular Ethernet MAC address and require "rebooting" to attach to a different MAC address.
Next I plug in the router into the cable modem/DSL with an Ethernet cable. Power on the modem. You may need to "release IP" or "DHCP renew lease" on the *router* if you don't get a new IP assigned to the router after a few seconds.
On the Mac, open the System Preferences and go to Network. Click on the Ethernet item in the list and click "Configure..." to check it's settings. You may need to click on the TCP/IP tab and click "Renew DHCP Lease" to get a new non 168.* or 10.* IP address (self-assigned addresses usually start with these numbers). Your IP address should be 192.168.x.x generally.
Now open any web site in the browser and see if you are on the Internet.
If so, go back to the router setup and check the WLAN (Wireless-LAN) settings. If you are using a mixed Mac/PC environment you may not be able to use any security setting above WEP because of the PeeCees or really old Macs. Always use WPA or WPA2 if you can.
Set a passphrase DIFFERENT than the admin password for your wireless network.
Set a wireless network name (SSID) to something unique (typ. one word, no punctuation). Broadcast SSID should be on at first so your wireless computers can show you if they "see" the network or not. After initial setup you can turn SSID broadcast off on the router.
Go to the wireless computers and see if the network shows up (in Mac OS X you can pop open the AirPort menu item and see if your network name is there, if not close the menu and re-open it). On windows you do whatever you do to "see" wireless networks with whatever software you installed.
Connect to the wireless (or select the same security setting as set on the router i.e. WPA2 Personal), enter the password and see if you connect. Lastly try to get on the Internet via web browser (easiest).
NOTE: you may need to reset the router or modem by pulling the power and plugging it back in if you cant get an IP address other than 168.* or 10.* on any computer or can't get onto the Internet, respectively.
IF you need to reset the admin password on the router you typically unplug the power to the router and press and HOLD the reset button for several seconds, then power back on and log in with the default user name and/or password for your router. Don't forget to RESET security settings, esp. for wireless such as remote admin, if you do need to reset the admin password on the router.