setDefaultController

Can i user DefaultControlle in subfolder like $routes->setDefaultController('Frontend/Home');

I'm afraid that's not how it works. You put Home as the name, and it will look for all Home controllers in e.g:
Frontend/Home
Backend/Home
Folder/Home

If you just specify /frontend, /backend, or /folder in your URI.

https://codeigniter.com/user_guide/incom...irectories