When set to Auto, the RUTX50 modem usually prioritizes connection stability over peak performance. If the LTE signal quality is stronger or more consistent than the available 5G signal, the router will stay on LTE even though 5G is technically available. This behavior is also influenced by the ISP’s network configuration, SIM provisioning, and whether the 5G deployment is NSA (which still depends on an LTE anchor). Clear details on this thread: RUTX50 - Locking 5G-NSA CA bands manually
Manually selecting 5G bands forces the modem to attempt a 5G attachment, which explains why it works only when configured explicitly. Firmware, supported band combinations, and local network conditions all play a role.
This is similar to how some games or engines (like Jump Force MUGEN APK builds) default to safer or more compatible settings unless higher-performance modes are manually enabled. The system chooses what it considers the most stable option unless told otherwise.