What is a proper way to consider the supplementary materials as a reviewer?

Your job is to provide critique and suggestions by which the authors can improve the paper. The published paper should be self-contained; that does not mean that as a reviewer you should ignore the supplementary material.

It sounds to me like you already know how to advise the authors to improve their paper. I would definitely ask them to make improvements and submit a revision, rather than reject — especially if the paper would be "perfect" with your suggested changes.

You say that a paper should be self-contained and understandable in its own right, and as this paper isn't there could be grounds for rejecting it. However, it's not the case that you've been sent a flawed paper and have no evidence that the authors can do better. The supplementary material demonstrates that the paper could relatively readily be improved to make it self-contained and understandable.

As Dylan Richard Muir has highlighted, one of your roles as a reviewer is to provide guidance on how a paper could be improved. Take this opportunity. Accept the paper (if the only options are accept or reject, as your question suggests) but strongly suggest that the authors improve the manuscript by bringing the relevant content from the supplementary materials into the paper.

If the authors choose to ignore your advice, and it's far from obvious that they will if the material exists in some form and there's enough space in the paper for it to fill, then it's really only their paper that suffers.