GPS accuracy?

what is the maximum accuracy I can use with the GPS call below

lpp.addGPS(1, GPS.latitude(), GPS.longitude(), 2);

If its to 5 decimal places meaning 1.11M accuracy (±0.55m)

is it possible to get further accuracies? or this cant be done using Cayenne mapping currently due to the configuration?

I’m looking for accuracy within 1mm using RTK