Diarrhoea & nappy rash

Hi mummies! Pls help to advise as my baby is currently 9 mth having bad diarrhoea which caused her nappy rash worse. We had try and error for formula such as isomil, s26 gold, enfamil for soy milk but baby still having diarrhoea. She had been taking similac since birth and breastfeeding(lasted 3 mth as insufficient milk) For solid food, she is doing well. Any advise for this? Also nappy rash, we had try pureen, drapolene & desitin.