Duo queue restrictions?

Hello lads, I have obtained a rank of diamond 5 euw, and I had this question: I have seen a post about 2 months ago, about dynamic queue changes about how gold can now only duo with silver and plat etc. But for diamond players the restriction changes: diamond 5 players can duo with..? With who!? I can't find the post anywhere, I cannot duo with my plat 4 support main, neither I can duo with my best friend who is diamond 1. Does anyone know the ranked restricions for diamond and higher? Help please;/