I am quite sure that according to Reed-Kellogg, the word "that" is never a conjunction. The word "if" certainly can be.
"That" can be a relative pronoun, a demonstrative adjective, a demonstrative pronoun, or a "function" word, but not a conjunction. True, "funtion word" is not one of that practically sacred number of EIGHT parts of speech, but interjections are suspect in that they are not bound by syntax, and expletives are also a little different.
But, if it comforts, one to think of this "that" in a group with "because, when, before, as," etc., then ok.
It is simply not what Reed-Kellogg would say. And the original request was how to diagram it. This "that" has a very special job to to. It introduces noun clauses and does not need to be there.