What does the phrase "in order to" function as?

"In order to" is a subordinating conjunction. (I.e. a clause that follows "in order to" becomes a subordinate clause, which needs a main clause to make a complete sentence.)

"In order to" generally introduces a "final clause," which is a clause that states a purpose.

The subordinate clause introduced by "in order to" is an adverbial clause, but the phrase itself is not categorized as an adverb.

A discussion of "however," a conjunction that can be an adverb, may be of interest to you. If so, take a look at this blog post.

References:

"In Order to" in English Grammar Today

"Final Clause" in Fowler's Dictionary of Modern English Usage