Backup,Format, and Reinstall windows, and be done in a couple of hours, or try to fix it for months, and get no ware.
Modems not working usually means malware, or virus.
The last time this came up for me, I did the cleaning, and scanning, for the user, and the on-board Ethernet still did not work.
I installed a new Ethernet card, and she ran it for another 6 months.
When I finally was able to do a clean install, the original on-board worked again.
sorry, but that's my experience, but I have only been doing this for 30years.