Hi @Jabez and welcome to the QuickSight community!
From my understanding, and according to the linked page, you should be able to use parameters with the netWorkDays function.
Would you be able to share screenshots of the 2 parameters you’ve setup for start and end date (preferably the view when you hit ‘edit parameter’).
Additionally, have you tried changing the aggregation in the field well to see if that changes your results?