The best workaround so far is to have a screen stating that their account is currently limited, nothing more. Use language like “visit our website for more information” and direct them to a webview of your website where they can then pay through your website. This has worked for a handful of apps, but I cannot guarantee this loophole. This is how Netflix and other companies get away with it as well. You have to signup on the web, then you get access to the app.
It’s very important that you mention nothing about “subscribing”, “premium”, “free trial”, or different “plans”.