Donation (V2) pages, by default, hide the top nav to allow for a more simple, focused UI that lead to higher conversion rates. If you would like to retain the top nav on a Donation (V2) page, you'll want to follow the instructions below.
1. Create a custom theme if you haven't already as we'll need to modify one of the theme files here.
2. Once you have a custom theme all set up, open up the nav.html file within the theme files.
3. In this file, you'll simply want to delete the first and last lines. The 'unless' conditional statement here is what prevents the top nav from being rendered on a Donation (V2). Once deleted, hit 'Save and Publish Change' to make the changes live.