Thus, a UE supporting both an LTE and an NR modem may operate in an LTE only mode (e.g., FIG. 11A), a non-stand-alone mode (e.g., FIG. 9B), a stand-alone mode (e.g., FIG. 11B), or with simultaneous connections to LTE and 5G NR (e.g., FIG. 11C). However, the UE cannot support both non-stand-alone mode operation and stand-alone mode operation at the same time, e.g., as illustrated by FIG. 11D. As shown, since the UE has a single NR modem 946, the UE cannot maintain connections 920 and 1130. In other words, if a UE connects to both LTE and NR cells independently (e.g., as illustrated by FIG. 11C) and eNB 904 attempts to configure the UE in non-stand-alone mode, the UE may reject this configuration which could lead to an LTE connection failure, e.g., as illustrated by FIG. 12. As shown in FIG. 12, UE 940 may establish connection 1134 with gNB 906 and connection 932 with eNB 904. Upon connection 932 being established, eNB 904 may attempt to re-configure the UE 940 for non-stand-alone mode via re-configuration message 1202. However, since the UE 940 cannot support non-stand-alone mode due to already established connection 1134, the UE 940 may reject the configuration and cause failure of the connection between the UE 940 and eNB 904 at 1220.
