Star Health - Claim rejected

My relative has lung cancer. This was detected in January end after doing a biopsy and scan. We submitted the same to Star for reimbursement. They rejected the same, saying they are unable to ascertain the duration of disease. Doctor has mentioned that the patient was coughing for 6 months, which is true as she had a light, dry cough for this duration. Star is asking for medical prescriptions of these 6 months and denying claim on the basis that we are not providing the required documents. The problem is that she hadn't gone to the doctor in this period. So, there are no such documents. After this, we have gone for her surgery. And Star is rejecting this as well. Can someone tell me on what is the correct way to escalate this?