I finally inserted the DOI in the Article reference of my paper. The method was to undelete the preprint reference, to remove the DOI from it, to delete the preprint reference again, and to insert the DOI in the Article reference.

But now comes another consequence of the fact that ResearchGate does not provide any way of converting a preprint reference into a published article reference: the statistics on the preprint (no. of reads) were not added to the statistics on the article. It is better to avoid including preprint references in a ResearchGate account.