A 68-year-old man with known heart failure (NYHA Class II, LVEF 35%) is currently treated with Ramipril 10mg OD, Bisoprolol 5mg OD, and Furosemide 40mg OD. He remains mildly symptomatic with exertional dyspnoea. His eGFR is 55 mL/min/1.73m$^2$ and Potassium is 4.5 mmol/L. According to NICE NG106, what medication should be added next to improve prognosis?