Please note that you wrote to the ResearchGate community, not to the RG team. See https://help.researchgate.net/hc/en-us/articles/14293044585745 for DOIs generated by RG. However, these are available for unpublished research only (and as far as I know, RG regards theses as published research). Note also that a DOI is not compulsory for citations. And see the following discussions for recommendations how to get a DOI: