Ant+ looses connection when starting a workout (Galaxy tab S7)

I woukd like to use Zwift. I followed all instructions.I use Bkool SmartPro2 trainer with the latest software of 3.25. When I start the Zwift App on my tablet I can connect via Ant+ to my trainer with choosing the Fe-C connection. I start with Power, then go to resistance, then to Cadence and finally heartrate. All is working smoothly and stable.
However, as soon as I start any workout the connection gets lost and the avatar is not even starting. Once I was lucky and could ride for 5 min, but the resistance did not work and then the connection got lost totally. Sometimes I can restore the connection with going back to the menu, but as soon as I start the workout the same disconnection happens. Sometimes I cannot reconnect and I have to re-start the App.

Ant+ dongle is only a meter away from the trainer, I disabled bluetooth and in the cellar are no other electronic devices.

Any hints are appreciated.