Recently, our app was rejected by the App Store citing "Policy 3.1.1" which relates to external mechanisms for purchases or subscriptions within the app. The App Store has requested that we remove the "user registration" feature from our app. However, this feature is essential for our startup.
Our app serves as a point-of-sale (POS) app and is an integral part of our main app (website) used by retailers and business owners. Currently, after users register, there are no further in-app payments required as payments are made directly via our website. Additionally, the mobile app does not contain any links or processes for in-app payments.
We are seeking advice from the community on how to effectively address the App Store's concerns and have our app accepted. Have any of you encountered a similar issue? Any recommendations on how to communicate with the App Store in a way that will help us resolve this situation would be greatly appreciated.

Thank you in advance for your assistance!