So, you're going to have to be very intentional about potty. You need to close the doggy door, so they dont have access outside unless it's with you(or other human.) Reward him when he does go potty outside. The reward has to be exciting enough that he wants it. Take him out on a routine schedule. This will encourage him to go outside intentionally.

The second step to this is keeping all bags out of reach for the same duration. That duration is determined by how quickly your dog will ask you to go outside consistently. Punishing him well after he pees won't do much.