Lyra is 22 meters above the entrance.
The radios work until Donna is 50 meters away from the entrance.
At that moment, Lyra, Donna, and the entrance form a right triangle.
The hypotenuse is the actual distance between Lyra and Donna.
 Distance = √ (22² + 50²)
 = √ (484 m² + 2,500 m²)
 = √ (2984 m²)
 = 54.63 meters.
Rounded to the nearest meter,  Distance = 55 meters.
THAT's the maximum point-to-point range of the radios.