Eureka Forbes — Brand new Water purifier not working. Eureka Forbes sending fake invoices

We purchased new Eureka Forbes water purifier online but when we installed it 2 weeks back the TDS was showing above 400, the installation engineer came 2 days after and checked it again but TDS was still same.
He said the RO is faulty and asked us to raise complaint with company for replacing the unit.

So far we have raised 3 tickets on multiple days with Eureka forbes but nothing is been done.

2 days back we received an invoice saying this is against the payment we made post installation which never actually happened.

I am not sure what to do now.