DHCP wan IP address won't change

My surf soho uses DHCP to get a wan IP from my ISP. The ISP says I should get a new on every day or two but that some routers want to grab the old one over and over. I have tried rebooting the router and doing a complete shutdown and power up and I still get the same WAN IP. Is there a way to force the surf soho to let go of the current one and except a new one?