ResearchGate assigns DOIs only to unpublished research (see https://explore.researchgate.net/display/support/ResearchGate+DOIs). For possibilities to get a DOI for a published article, see:

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

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

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

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

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

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

But note that DOI is a Digital Object Identifier, which is given only to digital files. It cannot be given to an article which is printed only and is not publicly available as a file. See also:

https://www.researchgate.net/post/Is_DOI_compulsory_for_Published_paper

Thank you so much for the help and the detailed information.