A 35-year-old man from Delhi presents with a 3-week history of purulent nasal discharge, facial pain over the cheeks, and fever. Nasal endoscopy shows pus in the middle meatus. CT PNS reveals opacification of the maxillary sinuses bilaterally with intact bony walls. He has already received 2 weeks of oral amoxicillin-clavulanate without improvement. What is the most appropriate next step in management?