Severe issues when integration third-party SDK in iOS specific app part

We would like to integrate Datatrans as Payment Provider into our Flutter app. Datatrans already provides a SDK for native iOS and Android apps.
But, so far we are not able to integrate the SDK into the iOS part of our app. We added the dependency via Xcode’s "Add Package dependency“ feature. This step passes without troubles. But as soon as we want to reference the SDK, Xcode states some errors: it’s not able to resolve the classes from the SDK and the import statement is marked with „error extracting version from module interface.
For testing purpose, we created a new SwiftUI project in Xcode and did the same steps there, but here we were successful.
Could anyone provide some tipps here? Does Flutter so something different regarding the platform native parts, so that we can’t use third-party’s SDKs as usual?

Any help is highly appreciated. Thanks in advance