Actually you need to do something more than just assign head page to the main page.
In your main page you need to create a reference - to the included content page.
You can do this by adding a “Content Page Include” in your main view, with reference to the content page
It will generate code like this:
<link rel="wappler-include" type="content-page" href="page2.html" path="/page2/">
You can even enter there the routing path so it automatically creates a route for your content page on save.
Note - this is actually done automatically when you use the “move to content page” action on the App Structure toolbar.
But if you have created the content page manually and assigned the head page, you need also to add the Content Page Include component - exactly on the place where the content page will be injected.
This all will help the data pickers render the data in the main page and inject your include in there, so its data is shown as well.
Hope this all make sense