Cannot connect to my network via ethernet cable, what can I do?

Well, when all else fails, I suggest reinstallation of the OS from the Recovery Panel (Command + R); your personal data will not be disturbed.

Although a safe procedure generally, there are sometimes problems so there is also the option of having Apple do a hands-on evaluation. An in-store diagnostic test is free. You can also do your own diagnostics: Use Apple Diagnostics to test your Mac - Apple Support

"does not have an IP address occurs when no Router will talk to you and give you an IP address. It is usually caused by simple problems like broken wires in cables and dead ports on Routers.

However, there is one case where this can happen that is not intuitive. If you have set a manual connection speed (e.g. Gigabit Ethernet) but that speed can not be achieved with your current cabling, the connection can not be made, so you would get no IP address.

The suggestion is to check to be sure you are allowing the Hardware to select its own speed automatically on this pane:

this picture was captured for a different problem.Just above the red arrows, be sure that "auoselect" is the setting, or change it to configure everything automatically.

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while you are up, be sure that on the TCP/IP pane, IPv6 is set to "link-local Only" not on or off.

Verify all connections are solidly in place, try a new cable, power cycle your router (assuming you have one), power cycle your modern, if using an adapter try a new one, insert cable into a different router port, insert cable into different Mac port ...

Lack of detail can often lead to suggestions you try things you've already tried.