Cashless hospitalization rejected - Fortis Hospital not in network list

I have a health insurance policy from Niva Bupa with sum insured of 15 lakhs. Last week, my mother had to be admitted to Fortis Hospital in Kolkata for severe pneumonia. When we tried to get cashless treatment, the hospital said Fortis Hospital is not in the network list of Niva Bupa. But when I bought the policy, the agent told me that Fortis Hospital is covered. Now I had to pay 4 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.