Questions about Garmin Speed sensor 2

The sensor is mounted on the axle of a wheel and counts the number of revolutions. It then calculates the distance travelled in a certain period of time based on the set value of the circumference of the wheel. I see 2 problems with the spinning bike: 1. Is there an exposed axle to attach it to? 2. what circumference do you set? For 2. you could of course use the typical circumference of a road bike...

I have paired the sensor with a Garmin Edge 830 and use it with it. The range really leaves a lot to be desired. On the rear wheel for indoor training I constantly have interruptions (sensor lost/sensor found) on the front wheel everything works well, but unfortunately that doesn't help much with indoor training because the bike is stationary. I would rather recommend the Wahoo Speed. It works great.