what type is your modem? for it to function with a router, the modem should be in BRIDGED MODE. maybe thats the reason why you are not connecting. once it is on bridged mode, connect the router.

if it is pppoe, just make sure to obtain IP and DNS automatically, no need to force the DNS (meaning assigning numbers to it).. just type in the username and password.

just click apply at the bottom.

for a netgear router, all you need is the username and the password, when in pppoe mode.

goodluck!!