May I first ask you to confirm whether the APN is set correctly, and if not already, could you try setting it manually to see if that improves performance?
In general, the modem will automatically select the most optimal bands available in your area at any given moment. However, for testing purposes, you can try using the band lock feature to manually lock the device to specific LTE bands that appear to provide the most stable results. More information on band locking can be found here: