t25 modem and asus router, dhcp problem

Just got the gigabit internet so upgraded to a arris t25 modem and using an asus ac3100 router, getting the message when my router is hooked up that the isp dhcp isnt connecting. Cant get wifi and will only get about 18mbps when hooked directly to the modem.  tried restartign and resetting and activating evrrything and same problem persists. anyone have any insight?