You are mixing 2 different things. The RMS in Reachview doesn’t say anything about the absolute position. It says about the spread of the current solution. Lack of spread in the RMS isn’t equal to high confidence in absolute positioning confidence.
Another thing is that even when fixed, you still need to add the receiver-deviation + ppm per baseline to your STD. At a baseline of 10 km that usually equates to low 1-digit cm Standard Deviation.
With RS+, you should only rely on the Fixed solution. In the survey-tab, keep an eye on the PDOP. If above 1.8-2, you are heading for trouble, if below 1.4-1.5, you are good to go (in general, “your milage may vary”). These numbers are quite low, and demand quite good sky-view.
If you only get a fixed solution 30% of times, you need to rethink your choice of positioning method/technology.
The RS+ shines in areas where you don’t have obstacles within ~30 deg (from the horizon) dome above the receiver.
If you primarily are working suburban or areas with (mild) tree-cover, you should consider the multifrequency RS2.
If you have heavy tree-cover, or lots of very tall buildings, even a multifrequency GNSS solution might not be enough for cm-accuracy. That would be total station territory.