why cant the boys keep a singal fire on the mountain anymore? how do they solve tht problem?

They are afraid of the beast. The boys decide to keep a fire below by the beach.

The believe that the Beast lives on the mountaintop. So, as Alsan says, they decide to keep a fire on the beach. This will not show as much smoke for a passing ship to see, but it is something. Besides, a hearth gives the beach a feeling of safety and of a "home."