Comminuted fractures of the tibia may involve the joint. This may cause stiffness of the joint. The degree of freedom of movement is not dependent on the x-ray picture. Sometimes the x-ray looks great, but the joint may be stiff. Sometimes, however, the x-ray looks terrible but the movement range may be good.
All that you need to get checked clinically is to exclude any mechanical block to movement or any associated soft tissue damage that may impede mobilisation.
At the moment all you can do is to persist with physiotherapy and if there is no change in six weeks, then get a review examination.