Rebooting the modem/router to get a new IP depends on a couple of factors...
1) type of service - cable/dsl/fiber/hot spot/etc
2) Dhcp release times on your provider's servers
3) (mostly for businesses)...maybe you have a static ip address, which wouldn't change, no matter how much you reboot
4) type of devices (modem/router, or modem and router)
Again, the advice of 'reboot the router', although mostly accurate, will not always work, depending on the above scenarios....but it is always worth a try, if you are not sure.