I assume you are on-premises, correct? If so, you need to run the database seedscript to make sure that iOS13 is recognized. These scripts can be found in the Resource Portal. The enrollment process is consistent with iOS13 and other versions so it should work but UEM needs to recognize the OS and for that you need to run the DB seedscript. Also, 9.3 went out of support in September of 2019, I would highly recommend upgrading to a more recent version as soon as possible.
Hope that helps!