If you have secured a database, and now want to split it into frontend/backend, you shouldn't use the database splitter wizard to do this.  If you do, you'll find the frontend will have the permissions you set, however the backend will be completely unsecure.

Instead split it manually:

1. Make a copy of your database - this will be the backend.
2. Open the backend and delete all objects except the tables.
3. Open your original database and delete the tables/relationships.
4. Go to File, Get External Data, Link Tables and locate the backend.
5. Choose all the tables and link.