|Unless your ISP made changes to prevent you from using a router with your connection, then the usual process is:|
1) make sure the router is set to use DHCP for the WAN or Internet connection.
2) power down both, the router and the modem. that means completely remove any power to the modem.
3) power the modem back up, allow a minute or so for it to "boot".
4) with the router's WAN or Internet connection port plugged into the modem's Ethernet port, power on the router.
If the router doesn't get connected to the Internet then you can try accessing the router's setup interface and see if there's any clue as to why not. Otherwise, you will have to ask your ISP to help you out.