iOS run problem

I’ve developed an institutional app using IONIC 3 (with CODORVA) that makes GET/POST requests to load info through API developed in C#.
Apparently everything was ok and all the tests were running right, but when we generate the version to publish into iOS, it was rejected by Apple’s validation due to a problem related to an IPV6. I did all the procedures to adjust in API’s server and the app was approved, however, when it is opened in any device, info generated from requests are not loaded.
Running the test in any browser, it works. The problem only persists in devices and iOS simulator.
I’ve tried most standard actions I could find and I still got no solution.
Has anyone been through something similar?

Additional Information:

I already enabled the CORS extension for API
Everything works properly on Android