I don't think you can renew your ip with ipconfig because the isp gives the ip to the modem and the modem gives the same ip to the windows by dhcp. If you use ipconfig the modem will give you the same ip because the isp hasn't changed the ip.
The only way you can renew your ip is to disconnect the modem for a few minutes, like Makaveli213 said, so the isp release the ip you were using and when you connect the modem most likely you will get a different ip.