Cashless hospitalization rejected - Apollo Hospital not in network list

I have a health insurance policy from HDFC Ergo with sum insured of 15 lakhs. Last week, my son had to be admitted to Apollo Hospital in Delhi for severe pneumonia. When we tried to get cashless treatment, the hospital said Apollo Hospital is not in the network list of HDFC Ergo. But when I bought the policy, the agent told me that Apollo Hospital is covered. Now I had to pay 7 lakhs from my pocket and I am trying to get reimbursement. Has anyone faced this issue? What should I do? Can I file a complaint somewhere? The TPA is also not responding properly to my calls.