it depends on what the complex “pro” features are you can not provide in your mobile app.
If they are only gimmicks but the main functionalities are all there i would not refer directly to the website in your app context.
Leaving the app context is not a good ux-behave, but it can not be avoided in any cases.
Example: We have built a shopping app, where you have a short description of a product in the detail view (because other text were to long and so on). So we added a “more info” button under the Detail-View, but it was really “visible” on the page… so any use was clicking on it by default, because… “hey there is a light green button on that page”.
So we added after the first click on that button a little popup with the message “that you will redirected to the product website in the mobile browser” and a checkbox where you can hide this popup (set localstorage flag to not show it again).
In other complex apps you have something like Tutorial-Overlays and Walkthroughs, but i think… this is for small apps a little bit to much. Even i handle it so --> the app have to be self-explaining.
In you case, you should think about, if this feature is really needed. If not i would not link it in the app.
And you can explain app users, that this is the app for you app, with the main-features and so on.
If there are additional features you need authorized for… i would not link them, because if they are not logged in on there phone and the browser they have to login first on the website and so on… thats not a good solution. So do not let the user see the feature.