You can try two things
First go into your router and release the wan address (don't renew it) and leave the modem unplugged long as possible, like overnight, but during the daytime would probably be better. The plan is to get that address back into the pool but most importantly picked up by another router.
Second (and you might want to try this first) Unplug the modem go into your router, most routers will let you clone a mac address from one of your machines the routers WAN port. Do this and plug the modem back in.