Losing bluetooth connection after 3-4 minutes

When I start my Neo Bike Smart and connect it via bluetooth with the tacx training app on an android tablet everything wents fine. all sensors including the heartrate sensor of my garmin fenix 6x pro turn green. after starting an activity the bluetooth connection shuts down after ~3-4 minutes and the activity pauses. The sensors in the tacx training app - settings - device manager regularly switching from green to grey.

Anybody have the same issue or knowing how to solve this problem?