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 https://play.google.com/store/apps/details?id=com.evbadroid.wicapdemo

It many, show all secured data

Obfuscated Text already done. How to encrypt