Thank you very much for your time and your support.
You are right, SSH-Login is possible!
But still no function on the external USB port.
In the meantime i have learned to build OpenWRT from source and the strange thing is, with my build - based on GIT hash 69ce75fb12526e11f79618a7cbedb32335b1f2bc, pulled master branch at Dec 27 2021 8:51 pm CET - the USB port works for me.
I will keep my build and configuration unchanged in case you want additional information.