Citing supplementary material as a reference?
Usually (with journals that make supplementary material available via the article web page), I'd say "see supplement[ary material]" or refer to figures as "Fig. S23" or the like.
As you comment that the journal will not make the supplementary material available, I'd suggest to publish in a repository like arXiv or Zenodo and then refer to that location. Still making clear that this is supplementary material to your paper, e.g. "see supplementary material [24]".
Or state at the beginning of the paper that supplementary material is available externally, give the location there (as opposed to in the references) and then refer to it as supplementary material like you'd do for "normal" supplements.