We used treats to train ours, too. At first reaching hands thru the door with a treat in it, then opening it a bit so they could see us with great on the other side, then tossing a treat thru the door each way ..... and always much “doggie door praise”. doesn’t take them long at all to learn.
But beware .. we always verbally said “doggie door” and “good doggie door”. In the beginning one of ours was standing by the door - when I said “doggie door” he ran to the treat jar, not the back door - lol. But they are all doggie door champs and have been for years.