Hello,
Usually this type of behavior is caused by a stronger cell being available nearby, and that cell not having 5G capabilities. This can be mitigated by either locking to the bands only available in the cell with 5G capability, or for more advanced users, performing cell lock. Cell lock has an added benefit of not “losing” any bands during the carrier aggregation.
For example, let’s say Cell A has LTE bands B1 + B3 + B7 + n78 5G, and Cell B has bands B3 + B20. Since Cell B is closer and has a better signal, the router will connect to it. However, it won’t have 5G access. With the first (band locking) option, you could only allow router to connect to B1 and B7. This way it will need to connect to Cell A, as these bands are not available on Cell B. You will have n78 available in this configuration, along with B1 and B7.
However, when using cell locking, you can lock onto B3 of the Cell A using it’s cell ID. This way the router will aggregate all bands available on Cell A, thus increasing the throughput. It should be noted, that cell locking is not an official feature, and can cause stability issues, thus it is not recommended to use this in environments where stability is important or no physical access to the device is available.
This procedure has been described in the greater detail in this thread: RUTX50 - Locking 5G-NSA CA bands manually
Additionally, if your device still reports the connection as 5G (NSA), then it should be noted, that the current RutOS release has a bug, where only 3 bands are displayed. The rest of them can be seen by logging into the CLI, and running the command gsmctl -A 'AT+QCAINFO'. If 5G band is available in the list, please disregard the previous comments about cell locking.
Hope this helps!
Best regards,