Since you are able to connect to the SB from outside its local area network, it sounds to me like the SB is correctly set up for viewing through the Internet. So I would suspect that the problem resides in your brother's computer, or his router setup, or Internet Service Provider's access to the signal. Can he try connecting from another location in Seoul outside of his own ISP, such as from a library or coffeeshop that offers free internet access? Or better yet, from another computer at that other location. If those alternates do work, then perhaps that would isolate the problem to his computer and/or his ISP.
Thank you for the suggestion. He has tried several other computers with the same result, adding that his colleagues are accessing their own SB using the same ISP.
I am going to re-install everything (SB & router) again this weekend in hopes that something changes & he can gain access. Maybe try different/additional router as well.
Regardless of the outcome, I will report back.