I presume you have configured your internet service using a pc directly connected to your modem. If not, you probably need to do this first as most ISPs requrire this. Check with your ISP and get the Internet service to work this way first.
Now to the router use -
I am assuming that you are talking about the WAN port configuration.
First you need to log into the router with the default user name and password. Usually they prompt you to change this password but you don't have to do that. You should however change from the default password.
Now to the WAN port configuration -
If you have a dynamic public IP address from your ISP (NOT PPPoE protocol which is Point to Point Protocol over Ethernet), you should indicate that you have a dynamic public IP address. You should NOTselect PPPoE protocol unless you are told to do so by your ISP. PPPoE requires you to submit a user name and password which was given to you for PPPoE protocol by your ISP. If you do not have a PPPoE user name and password from your ISP you most likely have IP routing instead and you need to go back to the config of the router to make sure the router was not accidently configured for PPPoE.
Please do not confuse DHCP. Many do. Your ISP can assign you a dynamic WAN IP address (and most do). When you configure your WAN port, you configure for a Dynamic WAN IP address. Seldomly do routers call this a DHCP configuration because the ISP has DHCP configuration but the router WAN port does not, it has to be configured for a dynamic WAN IP.
Now for the LAN (private side) IP -
This is where you specify DHCP from router to LAN devices. This is commonly done in your type of application
My initial guess is that you have accidently configured your WAN IP for PPPoE - check that out.
ALSO - be sure to reboot the modem and router. Always reboot modem whenever you change from connecting to a pc vs connecting to a router.