How to secure third party api in app?

I have created a recharge app and lost the hack three times. How do I secure the recharge APIs in it?
According to myself, I tried all kinds of security. Please help me, I have suffered a lot.

Try to encrypt your api and set in Obfuscated Text or Call data from any secure database.

Use Sniffer Wicap

It many, show all secured data

Obfuscated Text already done. How to encrypt