Allow cross domain Single Sign-on on multisite

I had Domain Mapping but deactivate it due to an issue with wildcard SSL certificate. And use the built-in WordPress mapping option.

The issue is when user login to main site, they’re not automatically logged in to the other sites. Which make it impossible to comment on the mapped subsites.

For example, if user is logged in to example.com, they need to login again to example.org to have access. While they still have access the the non-mapped subsites(like site1.example.com).

The question is, how can I force Single sign-on for all the domain and mapped one?