AFAIK, for IOS, if you want your users to pay via 'web only', you cannot mention anything related to external payment within the app, you cannot have a payment flow within the app.
What you can do is, ask users to sign in and/or visit your web page from where you can convert users. You can run email campaigns etc. Or if your users already have a premium login by making payment elsewhere, that should be fine.
For the Android part, I am not sure if they have updated their rules. Mathway offers in-app custom payment flow. https://play.google.com/store/apps/details?id=com.bagatrix.m...
There are particular rules to how you do it. You can't promote it in the app. The app has to have some functionality for non-subscribers. You can't use the itunes subscription SKU functionality to delivery your subscription content (obviously, I hope).
You should also consider joining the Enterprise app developer program that Apple offers if you want to by pass some of the above issues.
Not to be overly pointed, but it doesn't seem like you have done much homework here. I recommend you get more familiar with the market you're about to enter.
In an Apple Enterprise Developer app, you can basically do whatever you want, but you deploy it to devices yourself, and there is a limit to the number of devices. Apple doesn't even look at these apps.
I suspect they reserve the right to police some things (for example VOIP over unlimited data) -- but I don't know that that actually happens.