Homing requires endstops. If you don’t have endstops just jog the machine to position the tool where you want the origin to be and send G92 X0 Y0 Z0 to rest the coordinates. Alternatively, you can move the machine to this position before powering on.
When installing Repetier Host, did you possibly install Repetier Server as well? I’ve had it take over the USB port and not let Host connect.
should that be done when first opening the program before loading the job, or after loading the job?
If you are asking about the G92, then you need to execute it before your job starts running but after you’ve positioned your router at the home position relative to the stock. There are ways to automatically add this g-code command to your g-code file in Repetier-Host (Config/Printer Settings/Scripts), and probably your CAM solution also has ways to add this code automatically. In addition if you have a display and are running in Marlin mode, the first item of the V1 custom menu executes a G92 X0 Y0 Z0.