How can the doctors treat chronic lumbago?

Depends on cause. Chronic low back pain (lumbago) can be treated with medications, therapy, home exercise and even injections. It depends on the history of the back problem, the physical examination and sometimes diagnostic imaging.
Several things. Pain medicines, anti inflamatory medicines, back excercises, heat, ultrasound, steroids either by mouth or injected into the area, and as a last resort, surgery. Often the problem is poor posture, so correcting that can be very helpful. Testing with x-ray especially MRI can be helpful to clearly define the problem.