Ooooh nice now it got a little hacky with the GL.iNet MT2500A, I added an usb to rj45 modem thingy wirth rtl85something chip, added the modules to openwrt, then I've "if up" the interface and it worked. The only issue is that the tethering interface is not used as fallback but as main path, so I had to disconnect it via gli interface, on issue i can just reconnect via gui.
What did I put into the usb ethernet? The houses shared glass fiber from a different provider, has 500mbit/s as backup.