My first guess would be the splitter: most USB-ethernet adapters expect to plug directly into the Mac.

And my second guess would be the adapter’s driver: check the make and model and check for a newer version.

But you already seem to have the troubleshooting process down. Good luck!

Please describe in detail, link by link, your connection to the Internet. For example: fibre from ISP outside your house to ONT, ethernet from ONT to port 1 on modem/router [make, model]… and so on until you get to your laptop & operating system.

And any history that might help. Is this a new installation? Has it ever worked? Any recent changes?