Finally I was able to solve the problem with little impact on my network/router but could stay on one SSID name for both network frequencies.

As I did before once I configured the BSSID of the 2.4GHz band taken from my router (popular AVM FritzBox 7590 router here in Germany; BSSID´s are the MAC adresses of the 2 bands and of course are different).
Also configured 2.4GHz band as before.
Just for clarification, I did that configurations before without success.

But the trick was, to disable "band steering" in the router. This function allows the router to change the band based on internally information which band is actually better for each device.
After a reboot of the Astroberry it take the 2.4GHz band. My critical devices in my betwork stay at 5GHz.

I will observe the behaviour in my network in the next days and hope it´s stable in this configuration.

Thanks to all for helping me and I have learned a lot!

CS
Hardy

Really glad you found a way to fix this, and thanks for posting your solution should others happen to experience your problem.