Sim (Sitecore Instance Manager) don't support Sitecore 9.1
The security database, is the core database, you can change the connection string, but you also need the new sc910.identityserver IIS web app, is this created?
Have you seen the Quick installation guide", How to install the XP Single topology on a developer workstation. sc910_security
This is what you need for a installation of Sitecore 9.1