Bummer
you could leave it in 2wd, jack up a back wheel and run the car in gear (front wheels chocked), that way you should (with the cable disconnected from the gearbox) see the Speedo drive spinning, if not then I guess you are the first to have one break. If it is spinning then perhaps you didn't get the square drive to slot right in, I had to physically pull the cable out a bit and use pliers’ to push the cable end home on the gearbox.
Good luck