Buy a silver desk bell and leave it at the door you let your dog out to do his/her business. Sometimes it even helps to put your dogs paw on the bell too.
I did this with my dog here & I rang the bell every single time before I let her out. She wasn't doing it so I stopped for a while and then suddenly she started ringing it and she has been ever since
So don't give up on your dog :). I would show a video of her doing it but she only rings it if we aren't in the room or if she can't see us. If we are there she just stares at us with "the look" haha.