Add the chicken pieces, chicken broth, lemongrass, ginger, garlic, fish sauce, and mushrooms to the slow cooker.
Cover and cook on low for 6 hours until the chicken is tender.
Stir in the coconut milk, red bell pepper, and lime juice. Cover and cook on low for 30 more minutes.
Remove the lemongrass pieces before serving.
Ladle the soup into bowls and top with fresh cilantro.