Syntus will submit a report to the Public Prosecution Department if you have not paid the full amount on the 28th day. You will legally be in violation because you committed a punishable offence; fare evasion. The CJIB (Centraal Justitieel Incasso Bureau) will then send you the fine.
On top of that, Syntus is owed the cost of the fare and any applicable increase(s) of that amount. Your file will be handed over to GLN Bailiffs to make sure Syntus receives the amount owed.