ResearchGate is also something like a digital repository. However, only unpublished research can get a DOI at RG. See https://help.researchgate.net/hc/en-us/articles/14293044585745-ResearchGate-DOIs for more information.

Authors can obtain a direct DOI link by uploading their article to ResearchGate and linking it to the publication's DOI, or by manually adding the DOI in the article details section. ResearchGate will then provide a clickable DOI link on the publication's page.