A 38-year-old man from rural Maharashtra presents with a 3-month history of productive cough, fever, and weight loss. Chest X-ray shows a cavitary lesion in the right upper lobe. Sputum smear microscopy is positive for acid-fast bacilli (AFB). He is started on standard first-line anti-tuberculous therapy (ATT). After 2 months of treatment, repeat sputum smear remains positive despite good adherence. Which of the following is the most appropriate next step in management?