You can simply upload your network diagram to explain the setup.
- Raspberry Pi now has a stable release and you login to LuCI at 192.168.1.1
- Here, you can configure AP client on the RPI to get a working internet connection (to install the USB driver).
- If your main router has the same subnet (make it short, LAN IP) with the RPI. Go to Network - Interfaces to change the br-lan IP. Make sure after saving the change, you need to open a new Tab, visit LuCI at the new IP address to commit the change. Else, it will fallback to 192.168.1.1
- Now you can use lsusb to identify the VID&PID for your USB to Ethernet adapter and install the driver accordingly (Go to System - Software). For most of the USB 3.0 to Ethernet adapter, install kmod-usb-net-asix-ax88179; will work.
- Other build for RPI that I suggest - Rpi4 < $(community_build)
Good luck!