The hard part is that offline profiles have a different way of modelling your domain model.
You can find more info here: https://docs.mendix.com/refguide/mobile/building-efficient-mobile-apps/offlinefirst-data/
But what can help you forward is to change the 'Default home page' of your Offline profile (in the Navigation Profile) to a new module within your app.
First create a new Module. This will be the module which is 'offline-first'.
Then go to Navigation of your project and select the Phone Offline profile. Change the Default Home page and optionally the role-based home pages.
In this way you can keep your current app as is, and don't have all the errors.
Hope this helps.
I tried to do what you said by splitting my app in 2 modules, one for online and one for offline, but I still have the same errors. I don't understand why it does not work since I splitted my app.