Downgrade NAS542 to Firmware V5.21 (ABAG.9) C0

It seems, that the new Firmware for NAS542: V5.21 (ABAG.11) C0 is no longer compatible with the Nas Starter Utility. The NAS os no longer visible in the Starter Utility.

With the Firmware V5.21 (ABAG.9) C0 it worked fine.

Since I am not using any Cloud options. And I have five NAS 542 at home:

Is there a possibility to downgrade to ABAG.9? Is there a manual how to do this?

Greetings

Florian