The Son's coming to the Ancient of Days is into the Father's glory
Cyril is answering his own question about what it means that the Son reached the Ancient of Days, explaining that it refers to entering the Father's glory, not to spatial movement.
What then does it mean, “He reached as far as the Ancient of Days” (Daniel 7:13)? Clearly the Son reached into the glory of the Father. And from where is it possible to see this? He himself said again next: “To him was given the honor and the kingdom” (Daniel 7:14). For he heard the Father saying, “Sit at my right hand until I make your enemies a footstool for your feet” (Psalm 109:1).
Greek
Τί οὖν ἐστι τὸ, Ἕως τοῦ Παλαιοῦ τῶν ἡμερῶν
ἔφθασεν; Εἰς τὴν δόξαν τοῦ Πατρὸς δηλονότι ἔφθασεν ὁ Υἱός. Καὶ πόθεν τοῦτο ἔστιν ἰδεῖν, ἔφη πάλιν αὐτὸς ἐφεξῆς· Αὐτῷ ἐδόθη ἡ τιμὴ καὶ βασιλεία. Ἤκουσε γὰρ τοῦ Πατρὸς λέγοντος· Κάθου ἐκ δεξιῶν μου ἕως ἂν θῶ τοὺς ἐχθρούς σου ὑποπόδιον τῶν ποδῶν σου.