Salesforce Alerted me that at the end of February we would be forced into the security update for "Secure guest user record access". I tried this in my sandbox and it seems to have some adverse effects. Queries for some pages in our portal always return 0 records no matter what. This is even after setting up a new sharing rule to give guest users read access to that object. It's strange that for some older custom objects (created back in ~2015) the update hasn't seemed to change anything. Guest users can still query and edit / create those records like normal. Am I just setting this up completely wrong? At the end of the day, will it no longer be possible for guest users to edit / create some custom objects? I have made no changes to the guest user profile before checking the "Secure guest user record access" checkbox.
Relevant page from salesforce does not make it clear how to give edit permissions. It makes it sound like read only is the only thing possible: https://help.salesforce.com/articleView?id=networks_guest_profile_best_practices.htm