wired ethernet conn

The other potential issue is not a bad modem, but that you didn't allow this modem to initialize with your Mac.

I suggest you try the following before replacing the modem:

1. Power down both the modem and the MacBook.
2. Connect the MacBook to the modem with an Ethernet cable and the USB-C/Ethernet adapter.
3. Power up the modem. Wait at least 10-15 mins to allow the modem to initialize.
4. Power up the MacBook, and then, check for Internet connectivity.

FWIW, I currently have both the Apple USB-C to Ethernet adapter and one from OWC that also has additional USB 2.0 ports. Both provide Ethernet connectivity when used with my 2017 MacBook.

I suggest that you double-check to see if Ethernet is at the top of the network connectors choices in System Preferences > Network on your MacBook.

Does Ethernet even appear in the list? By default, with the adaptor disconnected, you would only see Wi-Fi and Bluetooth PAN for these options.

Apple doesn't make a USB-C to Ethernet adapter. They sell one made by Belkin.

And, the old Apple made USB-A to Ethernet adapter is not a Gigabit one.

Thanks. I should have clarified that I got the USB-C to Ethernet adapter from the online Apple Store, but it was actually a Belkin product.