In NSA networking, X2 Interface self-setup between the 4G and 5G base stations falls.

The self-setup switch is not turned on. In NSA networking, X2 Interface self-setup between the 4G and 5G base stations falls if the self-setup switch is not turned on. This is because the switch must be enabled in order for the base stations to establish a connection. Other possible causes include the 5G and 4G base stations belonging to different PLMNs (Public Land Mobile Networks), cell setup failing on the LTE side, and the number of links established over the LTE X2 interface exceeding the board specifications. Reference: https://www.qualcomm.com/invention/5g/non-standalone-networking-5g-nsa-networks