When you spin the tire, it should turn on automatically, or show some MPH. My guess is that the magnet is not close enough to the sensor mounted on the fork blade.
It should be as close as possible to the sensor without hitting it. We are talking mm’s here. Move the magnet closer to the center of the wheel and move the sensor as well. That will move the magnet farther outward.
Try that, and then change out the battery if it doesn’t work. Moving it to the other fork blade as earlier stated might be good to try as well.
Mine goes to sleep if it isn’t used. So, if I just turn the magnet one time past the sensor, the display comes on and that shows me it is working. A fast spin shows mph.
It’s counting revolutions. It doesn’t matter where the magnet is on the spokes. The computer setting (2770) is going to be the same no matter where the magnet is on the spokes.
You just need to position the magnet on the spoke where it can get close enough to the sensor. It needs to pass within a mm or two. You may have to shim behind the sensor (where it attaches to the fork) to get it close enough to the magnet. A couple layers of tape can work well as a shim.
Something around 2770 or 2780 will give you a good measurement.
I believe that the sensor has a little reed switch in it. The magnet opens (or closes) this as it goes past. That means that the switch has to be aligned in the correct direction, as well as being the correct distance away.
The possibilities are:
Flat or faulty battery. Unlikely if computer is new, and if the display is showing anything at all.
Faulty computer. Unilkely.
Switch (sensor) is too far from the magnet.
Sensor is wrongly aligned with the magnet.
Wheel/frame is twisting when you ride, and changing the distance between the sensor and the magnet.
You have not fully completed the set up procedure for the computer, so it is not yet ready to read the speed.