edit:
after playing arround with the phone a little bit, I did figure out that all I want is working like a sharm.
the problem was that I did wait for tethering appearing at the wrong settings page under "usb-settings" -->usb-tethering,
but I had to activate ethernet-tethering under "Hotspot & Tethering".
so everything is working out of the box with all usb-lan adapter I have (RTL8156B, RTL 8154, AQC111U) on P4a and P7a!
brillhant, now I can have my phones dual sim modem connected to my main router.
thanks again to @userofgos , you made figure it out.