If you want to use strings in Simulink, particularly if you want to generate code, I would suggest converting to ASCII characters and performing operations in that domain.
double(',')
returns 44, so instead of using strfind you can simply look for 44 in the data and split it up that way.
Using ascii directly will help you avoid using characters in simulink, which is not easy to do. This will open up the ability to pass character data using simulink blocks where you can later recode into a string if needed.