Unfortunately unpacking ones new weather station, erecting it on site and crossing ones fingers is not all there is to it

Your Barometer Reading will always need to be checked.
Invariably this is also the case after batteries have been changed.
A good place to check as to whether your SL ( Sea Level) pressure is reading correct is against those Location near you. You can do that off our Network map:
Live Weather Map
and of course on a still night, the Metservice graphs.

To calibrate any condition in Cumulus, especially the Barometer, select Configuration on the menu and Calibration.
You can see from the screenshot, this station is dropping the Pressure Reading by -2.000.
Make your changes and select ok. Return to Cumulus Frontend and wait for the change - bottom left.