When you say 'the course does not show up' do you mean that the course is not visible to the Guest user in the list of courses at all?
The Guest should be able to see the list of courses after login. So check the course is visible and the category is visible too.
The usual order of things I check for with Guest issues is:
- User policies - are Visitor and Guest set to the Guest role?
- User policies - is auto login on or off ? (It should work either way, just behave differently given a link)
- Is Appearance > Allow guest to use Dashboard on or off? (should work either way,just behave differently; when off it should redirect to the home page)
- Is Plugins > Enrolment > Guest access actually really enabled ?
- Is the Show button on in Authentication? (Definitely is in your case)
- Is the Guest access enrollment method added to the course and also is it really enabled?
By default, guest is logged in the standard log as well, so that is also a possible source of information for troubleshooting.