>an App that is a mobile version of a website
App Review rejects apps that are nothing more than a website. You will need to differentiate it from the webiste.
>Do we have to use In-App purchase
If the subscription allows a user of the app to run code in the app that they could not otherwise run, then yes, you must use IAP. Given the comment above - you will either need to use IAP or you will be rejected for being nothing more than a website. You could check "reader app" (https://developer.apple.com/app-store/review/guidelines/#reader-apps) and see if you fall under that.
Thank you for your reply. Our app will have a lot more additional features than the website.
So, if our app enables users to access features(based on subscription level) that will be consumed inside the App, then we have to use IAP. Is this correct? Let's say, there are 10 features available within our app. User with Plan A will access only 3 features within the app and the features could be allowing users to use something within the app.
>...will be consumed inside the App, then we have to use IAP. Is this correct?
See the ASRGs Business / In-App Purchase 3.1.1:
"If you want to unlock features or functionality within your app, (by way of example: subscriptions, in-game currencies, game levels, access to premium content, or unlocking a full version), you must use in-app purchase."