the internet is available in places like that still and Watchtower.org and JW.org are websites that can be accessed freely by all. There is no man made system that can prevent the work from being done. Governments, laws, ignorance, death. Nothing.
And to answer your question abt thise who died before true worship was restored, have you not heard? There will be a ressurection of the righteous AND UNRIGHTEOUS.
Unrighteous people are those who did not have the opportunity whether it be geographically, or otherwise to properly learn about Jehovah.
Only wicked people, meaning people who ignore the opportunity to come to God, wont be ressurected. And no human can read a persons heart to know whether they are wicked. Gods job only. Is a person who decides to pretend they are not home when witnesses come to their door being unrighteous, or wicked? What would God think if you ignore his messenger?
Jehovah has an answer for all questions u have the ressurection being the biggest. That solves all ur geographical and dark ages, etc examples. LISTEN WELL, THEY ARE COMING BACK TO LIFE. Just so they can GET TO KNOW GOD.
You and I have a wonderful hope of never knowing death... because we live in the time of the very end. If one dies before the great tribulation, the ressurection hope for that one is valid. So say u have a family member who heard of Jw, but they were rooted so deeply in their faith, they rejected it. Jehovah then will consider that persons heart to determine whether they should recieve the ressurection. The kind of God he is suggests he will look for ANY excuse to bring a person back, even if they rejected his message.
Adolf Hitler could he recieving this reward. This is why Jehovah is just and loving.
However, if you are around when the great tribulation breaks out, and u have rejected his message, it will be like Noahs day, only the righteous will he preserved. Why? Because those who survive will be around for new instruction from God on how to deal with those who will he ressurected soon after. They will need to be taught Jehovahs ways.
Jehovah has a lot of great things planned for humanity.