I always have a dog door, I do not leave access to it when we are out and our gates are always kept locked. All my dogs do well with it and at night I have a high powered light, it gets turned off when we go to bed and again no access to the door.
I usually start by teaching them to come in from outside first, remember it is easier when you have other dogs using it... they really want to join other dogs for reward time... later new pup does not want to miss out at barking at neightbors or stray animals so often starts following them out the door. Different dogs can take different methods, I still go out with my guys at least four times a day (the Lhasa has to be carried down the stairs). I always give them a gentle push through the first few times and treat right away, later I stand on the otherside and call, our little guys want to be with us so bad.
I do have to say my newst girl for the last year will only come in the dog door and will bark or ring the bell to go out...she does not want to go alone. After close to a year she went out on her own yesterday...came in yapping her success and turning cirlcles, until treated. Today she again went on her own, I was ok with her taking her time as I have pads for Boo Boo (when you turn 16 and have health issues and no one hears you ring, you just have to go) and she does use those pads when needed.
I really think your pup will learn quickly...just be sure if they're not back in soon...check...our guys are usually engaged in some mischief!
Robbie, Boo Boo, Yogi, and Misty's human.
Poohkey miss you, monkey.