Sorry If my description of the problem is a little bit confusing, this environment is itself!
I'll clarify the matter: I've got one MDS, from where I can connect to every single domain, no matter if it's placed on north or south.
The RADIUS part you explain on your post is right but I want to add that FortiAuthenticator is talking to the AD, and also it allows you to authenticate via local users (user created locally into the FortiAuthenticator), at the same time.
Now that this part is "clear", I'm able to authenticate to every domain, both north and south, using my username created locally on the FortiAuthenticator, but when I access the MDS through my "remote" username, placed on the AD, I'm able to authenticate and log into the North Domains only.
I'll check all the communications again and see if I'm missing something,
Thank you very much for your reply!