At first, we should start by the UE initial registration in the network
Initial Registration of Dual Connectivity Capable UE
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An EN-DC enabled UE first registers for service with the 4G EPC.
[1] The DCNR capable UE sets the “DCNR bit” in NAS message #Attach_Request in “UE Network Capability” IE.
[1a] The UE sends the #RRC_Connection_Request message on UL-SCH.
[1b] eNodeB responds with an #RRC_Connection_Setup message on the DL-SCH.
[1c] The UE replies with #RRC_Connection_Setup_Complete. This message carries #NAS_Attach_Request.
The #DCNR bit in the “UE Network Capability” IE is set. This signals to the 4G Core Network that the UE supports dual connectivity with 4G-LTE and 5G-NR.
[1d] The NAS messages from the UE are signaled to the Core Network via the Initial UE message.
For more details: 5G RAN Training