Seeing at Christ's return Satan is destroyed, we know at that point he must have been cast into the lake of fire, not just locked up.
Satan being locked up happened at the Accension of Christ when Christ was given all authority and power. Of course, the demons, and his angels were still left free to cause mischief. And there was also communication between Satan and His angels while locked up. But Satan himself was constrained in his prison, so limited in what he could do. Christ triumphed over principalities and powers.
But at the end of the age, Satan is fully loosed and that is why the end of the last days is so terrible. What with all the lying wonders and false miracles, the beast rising out of the sea, the numbering of the beast, the war against the saints, the one world evil government, the antichrist figure, the false prophet, unholy trinity, etc...
You cannot read Revelation as a road map except in a symbolic generalized sense. IF you are going to make the millennial reign 'amillennial', you also have to make the other verses in that chapter symbolic.
Satanic Rebellion Crushed
7 Now when the thousand years have expired, Satan will be released from his prison 8 and will go out to deceive the nations which are in the four corners of the earth, Gog and Magog, to gather them together to battle, whose number
is as the sand of the sea. 9 They went up on the breadth of the earth and surrounded the camp of the saints and the beloved city. And fire came down from God out of heaven and devoured them. 10 The devil, who deceived them, was cast into the lake of fire and brimstone where[
b] the beast and the false prophet
are. And they will be tormented day and night forever and ever.
The above must happen at the fiery return of Christ in 2 Thess 2, as we read that is when Satan is destroyed, and consumed. So Satan is gone at the rending of the world before the final judgement.