Heat the vegetable oil in a large pot over medium-high heat. Add the celery, mushrooms and curry powder; cook, stirring, until the curry powder is toasted, 1 to 2 minutes. Add the chicken broth, coconut milk, 1 tablespoon fish sauce and 1 cup water and bring to a simmer.
Add the green beans and bell pepper to the pot, reduce the heat to medium low and simmer until the vegetables are crisp-tender, about 3 minutes. Add the dumplings and cook until tender, about 4 minutes. Stir in the lime juice. Season the soup with salt and add more fish sauce to taste, if desired. Top with sliced scallions and serve with lime wedges.