You're two thousand years too late. You need to look into the realm of mythology to find "the first isekai".

Greeks went in and out of the Worlds of gods willy-nilly. If you believe those places are still on Earth, then Dante's Inferno is very clearly someone leaving Earth to go to the (un)godly other world, of hell. That one even has Modern Isekai traits, as it has what should be powerful figures of that land all bending over backwards to glaze the MC who is a blatant author-insert.

But most people refer to "isekai" as in the modern power-fantasy-isekai with a set of common tropes, which while not always being present in every trope and form, once sufficient ones are alongside a few core traits, it is considered "modern isekai".