What do you do when you receive a damaged iPhone?

I bought an iPhone 12 in July from Apple website. It was a brand new phone. I only tried using it after the return window but it never powers up. So I go to Apple Store to service the phone and the Apple agent said they would replace the phone since it’s within warranty. A few days later I receive the phone with no repair. Apple claim the phone is damaged or modified to the extent it is out of warranty. I never made any modifications or damage to the phone I literally just took it out of the box and plugged to a charger. So I call Apple Support and they send me an empty box to ship the phone to them. Today I received it again with no repair or replacement. What do I do?