It seems you were using gpsd at some point, it should update from that automatically.
Once it is in “Receiver” mode for the location, you can’t do manual changes.
So either you get it to automatically update from gpsd or you’ll have to ask tech support to change it to manual again.
Data from any consumer GPS can show max. deviations of up to 50 Meter which is 150 feet.
If i am setting elevation of my receivers i rely more on manual setting which i gathered from different sites offering this based on Google Maps/Google Earth.
If you have the GPS module, that option disappears.
I use the manual input (without a GPS module) because my antenna is significantly higher than the USB ports where the GPS module would reside.
You can go with a smartphone by the antenna and collect the latitude/longitude and ground height. Modern phones have a few meters accuracy, and that’s plenty good.
However, since January 1st 2020, the number of planes that don’t have ADS-B/UAT dropped significantly, I saw only one or two small planes and a military C130 this last week that were MLAT.