It is radiation induced fibrosis which leads to limited mouth opening. Also because of radiation therapy salivary glands as well gets affected leading to decreased salivation. You have a saliva substitute like saliva sure which will keep the mouth moist and prevent tooth decay.
Next Steps
You need to visit a oral surgeon specialising in head and neck oncology who will help you with mouth opening. You can use practo to locate.
Health Tips
You also have various other dental accessories to help maintain good oral hygiene usually suggested after clinically seeing the patient.