What milk is suitable for my baby?

When she was born, she drank S26 all the way until 10 months old. Then she had diarrhoea so we changed her milk to Isomil (soy milk) but it made her constipated. We tried to go back to S26 during the day, and give her Isomil at night. At first it was ok, her poo was not too watery or hard. After she turned 13 months, she had diarrheoa and vomitted a lot, also lost appetite to eat. Not sure if it is a sudden reaction to the milk, or if she had stomach flu. We went back to Isomil. And now she has not pooped well, pooped just a little bit twice in 4 days. Please recommend formula milk I can try for my child.