I believe this is called an anastrophe or a hyperbaton.
For Wikipedia, a hyperbaton is a phrase being interrupted by the insertion of words not belonging to the phrase. Your example would meet this definition, as ad intrudes on the noun phrase altos montes. This is what Wikipedia calls the "original" meaning, as opposed to the more general one, a word moving from its position in the normal or natural word order to a different one.
An anastrophe is more generally defined as a deviation from the normal or natural word order (whatever that may be), but also it is often implied that an anastrophe only moves a single word. This would still apply in your example, because essentially only ad and altos have changed places.
Quintilian speaks of an anastrophe (and not a hyperbaton proper) if only two adjacent words change places (Inst. 8, 6, 35):
Verum id cum in duobus verbis fit, anastrophe dicitur, reversio quaedam, qualia sunt vulgo "mecum", "secum", apud oratores et historicos "quibus de rebus". At cum decoris gratia traicitur longius verbum, proprie hyperbati tenet nomen: "animadverti, iudices, omnem accusatoris orationem in duas divisam esse partis". Nam "in duas partis divisam esse" rectum erat, sed durum et incomptum.
So in this view, your example (which is analogous to quibus de rebus) would not be a hyperbaton.