Francis X Gumerlock (who is an expert in Patristic commentaries on the book of Revelation) refutes the perpetual myth that Chiliasm was condemned in early church councils; for example at the Council of Constantinople in 381 or the Council of Ephesus in 431.
He begins his essay situating this question by talking about how premillennialism was not the universal position of the early church, that a form of amillennialism was present as well. I think this latter issue receives important qualification that I can take up another day. But I want to draw your attention to the main part of his essay that argues that it is a historical inaccuracy to say that Chiliasm was condemned at any early church council.
You can read his well-documented essay here entitled “Millennialism and the Early Church Councils: Was Chiliasm Condemned at Constantinople.