ResearchGate assigns DOIs only to unpublished research, see https://help.researchgate.net/hc/en-us/articles/14293044585745.
Please note also that DOIs are not compulsory (see this discussion: https://www.researchgate.net/post/Is_DOI_compulsory_for_Published_paper/1), in ResearchGate you may leave the DOI field empty.
See also these discussions on how to get a DOI for already published articles: