How to solve battery problem

Did you read this link --> About genuine iPhone batteries - Apple Support

You bought a used iPhone. It's very possible the previous owner had the battery replaced with a battery which is not an authentic Apple Battery. If that is the case with your phone, Battery Health will not show.

First you tell us what problem it is you believe needs to be solved? We're not mind readers.

The only way to know for sure would be to schedule an appointment at your Apple Store Genius Bar and let Apple see the phone. They'll know if the phone has been tampered with.

I don't know. I don't work for Apple. Google Apple stores in your area.