Apple reject APP for not using "In App Purchase" for subscription

We have a simple application where users can post ADs and others can bid/respond to the ADS. The users can purchase premium plans to post more ADs per month or bid on more ADs.

We are using STRIPE to handle payments & subscriptions. Upon trying to publish to the App Store, we are getting the following error.

Guideline 3.1.1 - In-App Purchase

We noticed that your app contains a payment mechanism other than in-app purchase for digital content or to unlock features or functionality within your app, which is not appropriate for the App Store. In-app purchase is the only valid in-app payment mechanism for digital content.

Please suggest me the possible ways to solve it and publish on APP STORE. thanks!

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