ResearchGate assigns DOIs only to unpublished research, see https://help.researchgate.net/hc/en-us/articles/14293044585745-ResearchGate-DOIs for explanations and instructions. However, what do mean by "self publish a journal article"? A journal article is published in a journal, it cannot be self-published. Or do you mean that you want to create a page at ResearchGate for an article published in a journal that did not assign a DOI? In this case, see the reply by Anton Vrdoljak to https://www.researchgate.net/post/How_to_generate_an_DOI_for_publications.

Hello Jon Chang

If you want to self-publish because you need more publicity, I would also put it on my own website and try also to get it on my website for my work department or interest group or society.

There is facility of providing you with DOI here in RG, but I think this assigned DOI is local one!