It's a subordinate clause functioning as an adjunct. Please don't call it an adverb clause - that's a highly misleading term, especially in this case where there's no adverb at all in the clause. And it's a dependent clause, not an independent one.
It's certainly good to consider different ways of looking at things, and different grammar terminology, but at the same time my advice is not to rush immediately to abandon the grammar system that you have been taught before you have a good understanding of a new one.