I have to disagree, I'm 99% sure the DCNR flag is controlled from the provider side (MME) and I expect that to depend upon the particular plan a user has (I don't know what else would control it!?)
This GSMA document regarding upgrading from 4G to 5G, the MME in the core network should "Support 5G subscription access control (DCNR, Secondary RAT)"
This version of the document currently provides detailed guidelines for implementation of 5G using Option 3, reflecting the initial launch strategy being adopted by multiple operators. However, as described in “GSMA Operator Requirements for 5G Core Connectivity Options” there is a need for the...
www.gsma.com
View attachment 297
This Cisco document outlines what happens with the DCNR flags
"If the RestrictDCNR bit is set to "Use of dual connectivity with NR is restricted" in the EPS network feature support IE of the Attach Accept/Tracking Area Update Accept message, the UE provides the indication that dual connectivity with NR is restricted to the upper layers. "
5G Non Standalone Solution Guide, StarOS Release 21.9-5G NSA for MME
www.cisco.com
View attachment 298
So I believe a device will not even attempt to connect to 5G if it has previously received the Restricted flag, the messages that communicate these occur when connecting to the network and/or switching between cells. As NSA 5G
needs to have a 4G connection made before it connects, 5G wont even try connect until it has made a connection with 4G, by which point the DCNR flags will be set.