See these discussions for recommendations:

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