Well, the official tool for setting the RFD900 radios is shown here:
http://rfdesign.com.au/documentation/rfd900-config-tools/
While we are on the topic, I will give a shameless plug for SikSet.py, a new tool for changing RFD900 settings from Reach’s command line. One day, I hope ReachView will also be able to control the radio settings by utilizing this tool.
But this tool is something you would have to copy to your Reach and then run from the reach on the command line. It may just be easier for you to run the official Windows config tool with the radio plugged into your computer with the USB cable.
Back to the question: You want to run with GPS_Glonass_5Hz and the theoretical speed is 11584b/s. The next closest, but higher Air Data Rate (ADR) setting is 16 (or 16000b/s). You could try that, but I would recommend going at least one step higher to 32 (or 32000b/s).