It's possible that the network adapter is configured to use the settings from the previous ISP. Access Control Panel > Network and Internet > Network Connections and right click the network connection used to connect to the router. Right click it, then select Properties. Double click the IPv4 option and set the IPs to be obtained automatically. After you save the changes, you will see the effects immediately. You reboot the computer if you want, but it's not that really necessary. At this point, you should have Internet connection if the router is connected to the Internet.