Slow Cooker BBQ Chicken Pizza Soup

8 cups chicken broth

4 skinless

boneless chicken breast halves

1 (24 ounce) package frozen corn

1 red onion

diced

1 ¼ cups barbeque sauce

4 cloves garlic

minced

1 ½ teaspoons salt

½ teaspoon ground black pepper

½ teaspoon garlic salt

½ cup shredded mozzarella cheese

or to taste

½ cup chopped fresh cilantro

or to taste