Need wireless v28.17 after updating to v20.29

After updating to v20.29 the Forerunner 955 does not sync anymore to the iPhone. Trying to pair it again it now shows the message "Need wireless v28.17".

I have tryied to go back to factory configuration (system>reset) but it only deleted every app, data and configuration but it does not go to the old OS version.

How do I move on from here?