Based on your post, I understand that you want 2 things: gain weight and lose 'belly fat'

1. You're underweight, but you did a lot of fasting? once a week is ok for health purposes. But you want to gain weight but fast regularly?

2. Your workout sucks. All you're doing are ab exercises. You need to do compound exercise. squats, deadlifts, bench press etc

3. You can't spot reduce fat. Fat is burned all over your body, not in 1 particular area. Doing ab workouts will not help you lose belly fat

4. When you exercise regularly, especially lifting weights, you'll get hungry. No need for appeton weight gain

5. Lifting, especially compound exercise will help your muscles grow and help you gain 'healthy' weight. By 'healthy' I mean muscle, not fat. you don't want to add fat just for the sake of gaining weight