If network nodes (MME) supports 5G but just in HSS subscriber hasn’t 5G subscription, at attach request UE put DCNR supported and after MME send ULR to HSS and at ULA, NR as secondary RAT was not allowed, at create session request, MME doesn’t put DCNR flag.
But if MME doesn’t support 5G, after a few times attach, UE finds that network doesn’t support 5G and UE also doesn’t put “DCNR supported” in attach request.
At Samsung UEs, usually DCNR_RESTRICTION shows that network supports 5G or not.
If this value be true, UE doesn’t put DCNR flag in attach request based on my experience.