MY Linksys WRT54G doesn't connect to the internet through wi-fi?

The router is connected to an ADSL modem. I do have internet connection on my PC that is connected to the router via Ethernet cable, but when I connect to the same router through Wi-Fi on my iPhone or Samsung Galaxy phone it shows that they are connected but I can't access the internet.

Their browsers try to open the webpages but they fail after 10-15 seconds.
I did try to remove the security options from the router. It didn't help.
It's set on DHCP.

The Stock Firmware on those Linksys routers sucks really bad.

I recommend you flash DD-WRT on it instead because the Linksys firmware isn't very good with managing connections and it likes to crash alot where you have to power cycle it often.

The Version 4.0 and 5.0 and newer has very little ram in it which means it will only work with 512 connections at max. The older 2.2 and 3.0 can manage 1024 connections. Plus you get features that the stock linksys firmware does not even offer.

The other option buy a new router and one with gigabit interfaces to improve networking speed between pc's.

But you can try the DD_WRT here.
http://www.dd-wrt.com/site/support/router-database

Use http://192.168.1.1, http://192.168.0.1, or http://192.168.2.1 on the computer that has the internet working.one of them will bring you too a log-in screen type admin as the user and leave the other blank if the log-in fails type admin as the password as well. This will bring you to a set screen where you can name your Wi-fi and set up passwords and it's Wireless Channel witch is what I think it may be for you try changing it to a higher or lower Wireless Channel in your wireless settings.