USB-C adapter LAN port not working with MacBook Pro 2018 running Monterey 12.3.1

It looks like the connection settings in System Preferences->Network are wrong.

It could be something in your mac or your router.

By your description, my guess is that the router does not have DHCP enabled over Ethernet;

or maybe you are connecting to the wrong port on the router.

Some routers have 3 or 4 LAN ports and also a WAN port.

Make sure that you are connecting to one of the LAN ports.

Also make sure that DHCP is active on the router.

Can you post a screenshot of System Preferences->Network, with the Ethernet port selected?

This seems to confirm that the router is not acting as a DHCP server over Ethernet.

You will have to make changes to the router settings. Most routers provide access to their settings using a web browser, but they each have their own configuration pages.

If you have wifi access you may access them wireless and make the necessary changes.

On the other hand, to configure manually you need to set IP address, subnet mask, router address and DNS server.

The subnet mask is usually 255.255.255.0

but could be slightly different. The router address you probably know, and the DNS may be the router or one provided by your ISP.

It seems that Monterey does not have good drivers for that Realtek chip:

https://forums.macrumors.com › threads › macos-monterey-users-report-connectivity-issues-with-usb-hubs.2320450 › page-5

macOS Monterey Users Report Connectivity Issues With USB ...

There is a known problem with Realtek RTL8153 Ethernet chip which many docks use. Big Sir introduced DriverKit architecture and Realtek hasn't released DriverKit compatible driver for almost a year now. The default Big Sur Apple driver isn't reliable. I encountered this problem with 2 separate docks from 2 manufacturers. Both use RTL8153 chipset.

There was also a Monterey foru discussion on this same subject but I can't find it.

The router settings appear to be normal.