ResearchGate assigns DOIs only to unpublished research, see https://help.researchgate.net/hc/en-us/articles/14293044585745.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:

https://www.researchgate.net/post/How_to_generate_an_DOI_for_publications/1

https://www.researchgate.net/post/Getting_DOI_for_individual_papers/1

https://www.researchgate.net/post/How_can_I_get_DOI_for_my_paper/1

https://www.researchgate.net/post/How_can_I_get_a_doi_for_an_article/1

https://www.researchgate.net/post/Adding_DOI_to_documents/1

https://www.researchgate.net/post/Is_there_special_app_to_generate_Doi/1