You need to send commands directly to he GRBL controller. You can do this via CM or using a serial com package like PuTTY. I use PuTTY, as I am comfortable with G-code and the underlying hardware system.
The summary of what I did is Inaccurate positioning
I don’t normall chime in but I thought I would on this one. I would try what Will mentioned first off. Where I use our machine I have easy access to pretty high end optical measurement systems. When I got the Nomad I was having the same issues as you. Calibration was part of the issue. Mine wasn’t off by much (0.025") but now its is extremely accurate. I have run test holes 5" apart and the center to center stayed within 0.00005".
The biggest thing was fixturing and learning to work with the material. I gave up on anything but the threaded table a much clamping. I could never get the tape to be accurate. I’m milling plastics, so learning the materials thermal expansion, feed and speeds, and accurately measuring bits made the biggest difference. I now pull off +/-0.001" in Delrin daily on my Nomad with little issue.