I enabled "Secure guest user record access" in my sandbox & created a sharing rule on a custom object for the guest account. I am able to see records whose owner is set to my website's guest user account. The guest user cannot see records that are owned by other users.
To solve my problem, I created a sharing rule on my custom object. The sharing rule was comparing a network id custom field to a network id. When I had the rule say networkid=0DB6A000000UA3FWAW, the sharing rule did not work. However, when I used networkid starts with 0DB6A000000UA3FWAW, it worked.