I guess it would have been easier if the "Sign in" page is made available on the mobile feature list on the network dashboard so that moving it to top will solve this issue.
The mobile version will work in much the same way as your regular site. If you want your network to be private and force visitors to the sign-up page, you'll need to set your network to private.
Thanks Eric for the update, we don't want to switch to private, the problem is just that in the mobile forum discussion, groups, blogs..etc the comment/reply button seem to disappear making it impossible for guest to sign in/sign up. If the buttons can be made available on mobile so that once a guest clicks it, it routes them to mobile sign in page. Can this feature be added.