A Guest user is mostly associated with a Force.com Site. By default, the Site Guest User does not have access to any objects, fields, or VisualForce pages. You need to enable permissions and page visibility that are applicable to your use case.
Giving full object level permissions might not work in this case as Guest User Profile separately controls the permissions. So, you will need to open the Site --> Navigate to "Public Access Settings". This will open the guest user profile. Here, you need to enable the necessary permissions.
You can only assign the permission to read/create on standard objects, but can assign full create-read-updated-delete (CRUD) on custom objects. Please read https://cloud4good.com/announcements/site-guest-user/. Seems to be helpful.