Perhaps you didn’t read it
the Millennium is a spiritual reign of Christ that has been present in the Church since Pentecost. In other words, the Millennium is an ecclesiastical kingdom, founded at Pentecost.
The book of Revelations is a book filled with numbers as symbols - the “seven spirits” for instance - 7 is the number for completeness. Hence the 7 spirits = the one Holy Spirit of God.
Ditto for 1000 — that is 10 x 10 x 10 and in Hebrew repeating something thrice is for a superlative - hence the Holy, Holy, Holy and also the 6 6 6 = where 6 is 1 less than 7 so incompleteness