Help with Remote Access Router Settings AT&T and DD-WRT

I’m trying to set up remote access and am coming up short. I have at&t as my provider. I am using at&t’s router/modem with the router shut off. I use a dd-wrt enabled router to access the net. Could someone walk me through the steps to set everything up to be able to have remote access? I appreciate you’re help; I’ve come up short with google searches.