Wi-Fi working but Cable do not connect to internet

Hi, I have broadband connection with Modem. I can use it quite well with Wi-Fi but when I disconnect Wi-Fi and connect modem with laptop by cable- It shows connected with modem but do not connect to internet and cant open any website. Dell technical support  person said to ask about DNS setting to brodband service provider as drivers are ok. Broadband servise provider said DNS setting has nothing to do with cable connection as Wi-Fi is working properly. Can anyone help me? Thank you.

 Well...modem or router?  simply drop to a command prompt (DOS box) and type the following:

ipconfig/all

what does it state for an ip?  If it says something like 169.x.x.x then you need to renew your ip settings.  type ipconfig/renew and that should pull a new ip from your router.