I bought a Wifi-Extender with a Gigabit-LAN-Port. This Solution is perfect working. Its the D-Link EXO AX1800 Wifi Extender. Only 15 Euros at Saturn in Germany.
It works with every WLAN-Router - even the non Mesh-Models like mine. I had only to make the Setup over Cable in the Webbrowser-Interface and not the Smartphone-App.
Its working now perfectly. Its very fast (Wifi 6) and has no compatibility Issues, because i am using the LAN-Port now. And by the Way: It was cheap compared to a Wifi 6 (AX) Stick.
The Realtek-Chipset-USB-Stick i bought is not FreeBSD compatible at the Moment. Thanks for your help and advice.
I found a Solution from your Advice. Otherwise i could get mad without getting it running.