When you launch the Canvas Student mobile app, there are only two options available from the home/splash screen:
If a user is taking a course via Free for Teacher, there is no obvious option to select from. Since they may not be associated with any school, the first option doesn't make sense, and Canvas Network only pertains to course for educators.
Currently, the only way to for a student of free for Teacher to get access is to select option 1 above and then in the search box type "Free Canvas Accounts" and then select this option to get to the correct login screen (which by the way looks identical to the Canvas Network login screen which is confusing). This is not at all evident, and hence most students will have no idea to get to the correct login screen.
Instead, add a button option on the home/splash screen for Free for Teacher accounts, so that it would look something along the lines of this:
1. Find my school
2. Canvas Network
3. Free for Teacher
See screenshot example below (boxed in red):