I had massive problems when activating the shoe sensor.
Compared to all instructions from the Internet and the Bedienunsanleitung there is a trick that worked for me immediately. I was told by Niek + Support.
The button on the shoe sensor needs for 5 seconds, are best with a pen, pressed dannnach again rhythmically press 10 times, such as at a Herzrhytmusmassage.
Then in RUN on OPTIONS, NEW SENSOR.
There he was, recognized me immediately.
So if this does not help, I would complain about the clock.
GPS and heart rate measuring works great.