House Training an Adult Dog?

I have a year old Shepherd mix that I rescued from a shelter when he was about 5 months. From the beginning he has been more difficult to potty train than any other dog I have owned. He is fine when I am home (rings a bell at the door to go outside) and has never gone inside while I was home, but if I am gone, even for thirty minutes or more, there is always at least one puddle and sometimes poop.

I have tried kennel training him. He loves his kennel when I am there, but if left in it when I leave I come back to a kennel full of poop smeared everywhere. I have run out of ideas. All suggestions are welcome.

By Kari D