S-Function Builder in R2021a does not currently support symbolic dimensions.
If you want to continue down the route of using S-Functions, you will have to modify the generated S-Function to use symbolic dimensions. This doc page is a good starting point:
If you are open to alternatives to the S-Function approach, try the C Function block. The C Function block supports Symbolic Dimensions without doing any extra work, and it lets you specify Initialize, Start, Output, and Terminate code.
Here is relevant documentation about this:
