It uses a flashback technique as the conditions it deals with were found in the mid-1st century until the end of that century. Basically, what the author(s) of Revelation [there may be more than one author] was saying is that no matter how horribly they were being treated, "Jesus has your back", which is a message that still applies today.
Thus, the "millennial reign", which doesn't appear in any other Christian source btw, probably refers to Jesus' Kingdom here on Earth that was established with his creation of the Church and his ultimate crucifixion. However, his Kingdom also is found in Heaven, thus what's referred to as being the "communion of saints".
you really dont know what the kingdom is going to do ,, humm
what Isaiah has to say
''6 The wolf will reside for a while with the lamb,
And with the young goat the leopard will lie down,
And the calf and the lion and the fattened animal will all be together;
And a little boy will lead them.
7 The cow and the bear will feed together,
And their young will lie down together.
The lion will eat straw like the bull.
8 The nursing child will play over the lair of a cobra,
And a weaned child will put his hand over the den of a poisonous snake.
9 They will not cause any harm
Or any ruin in all my holy mountain,
Because the earth will certainly be filled with the knowledge of Jehovah
As the waters cover the sea.''
that has not happened yet .it will only happen in the kingdom reign of 1000 years