Wireless connection problem

Hi. 

I have Latitude E6410 and connect to internet through wireless network to Zyxel ADSL modem. My problem has started 20 days ago when I was connected then I was disconnected from internet. The status is as below:

- I can ping the modem which means the laptop the link between modem and laptop is working while I can not ping any address or website even the provider's DNS. At the same time, if I connect the LAN cable, every thing is working properly. I tried with different laptops and there was no problem. To resolve the problem temporarily, I have to restart the modem or change the IP for my wireless card. In this case it works for another 30-60 minutes. the issue is the same when I use different modem.

Can anybody help me to find out the solution?

Sorush.