On Debian 9, I use wicd and it works fine.

But wicd was completely removed from Debian 11 and 12. Now there is Network Manager and some nm-applet thing that replaces wicd.

And the nm-applet thing can connect to my modem/router over Wifi, but not over Ethernet. The option is just not there. I want to use Ethernet.

Is there some other application that will let me manage my connections as well as wicd, including Ethernet?

In my experience, if you have a valid ethernet connection, the OS senses it without the need of network manager thingees.

The first thing I would do is test the ethernet cable (or substitute a known good cable) and perhaps try a different port on the router/hub end of the cable.

If that doesn't help, I'd try booting to a Live CD/USB of something and checking whether the connection works.

Just a couple of thoughts.

Network Manager is perfectly capable of configuring wired ethernet connection, as long as the ethernet interface is detected and up.

To find out if it is detected, merely open up a terminal application, such as xterm, and type this command: This command means: the sudo part of the command elevates your priviledge from your regular user to an administrator who can change settings; the ifconfig stands for interface configure; the -a means show all interfaces. After you type this command, press enter, and then enter your password. If your ethernet interface is detected, it will come up in the list, usually as eth0. If, in the list of results, you do not see eth0 or eth1, or some other number after eth, then your inteface was not detected, and either there is hardware failure, or, very unlikely, missing kernel options. If it's a tower, with a pci card, perhaps ethernet card is not seated into pci slot.

If the interface is detected and in the list as eth0, then the interface is detected, and is probably just in a down state, which can be easily remedied with this command:
.
This command brings your ethernet inteface to the up state, but without an assigned interent address yet.
Since it is up, network manager will now detect it.

You can type at the same terminal:
A network manager window will pop up, with an option to "activate connection". Choose that, and there should be an option for "wired" connection, that when you activate it, will query your router and obtain an ip address for eth0.

Should you want to bypass network manager, it is almost easier. In same console, type:
This command requests the dhcp client daemon to request an ip address from the dhcp server in the router or modem your ethernet cable is plugged into, and that dhcp server will then assign an ip address to your ethernet connection, as long as one is available to assign.

Like I said originally, on Debian 9, I use wicd and it works fine.

Debian 12 is on the same computer, different partition.

I've tried the Debian 12 live CD and it can detect Wifi, but no Ethernet.

The Spiral live CD (just Debian 11 with tweaks) does the same.

I think Debian 9 works because it has wicd.

At the risk of sounding querulous, I must say that I have no machines that have wicd installed. They all connect to a wired ethernet connection with no issues and with no need to use any network manager thingees, including my Debian Sid box (though, I must say, I normally use it with wireless, but I have used it with wired).

Whatever issue this machine has, I think it likely that the absence of wicd is not it.

Aside: I think that wicd is an excellent application. It just happens that I have had no need to use it recently.

Just my two cents.

It is detected, it shows up as eth0 in 'ifconfig -a' but I forgot to say this Ethernet connection requires a password. It's not Wifi, it also has Wifi, but Ethernet needs password like a Wifi access point. There is no such option anywhere.
So much for "perfectly capable of configuring wired ethernet connection."

I forget the error message, but it was something like "no connection in that address."

But it didn't. It told me,

The command says,
(some time elapses)
It needs a password.
But Network Manager can't handle Ethernet connections with password.
Wicd can.
But wicd was discontinued because preserving Python2 is against someone's religion.
"It's too old!"
"It's DEPRECATED."
Here, use this. It doesn't work, but oh it's so modern. George Jetson uses it!

Are you using PPPoE / DSL for your internet service? That would make sense if you need to enter a password. If so you need to install rp-pppoe and use nm-connection-editor to add a connection.

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interesting, I have debian 12, network manager, no wifi at all and works flawlessly (without password).
I would recommend you to check your logs to see what's going on.
But if I remember well you use ubuntu packages on your debian, so probably your system is just messed up completely.

No, it's not PPPoE/DSL. The modem is always connected, but the modem requires a password for Wifi and Ethernet.

NO. God, no.

I use Ubuntu packages in Debian 9, and everything works beautifully in Debian 9, that's why I still use it.

Debian 12 is a brand new installation. I have added nothing to it because I can't even get the internet connection to work correctly. It's just sitting there doing nothing. Oh, but it's so new and modern. No Python 2! A beauty of masterful engineering.

Seems like there is no dhcpd service listening.

When it connected via wifi, do you remember the range of ip addresses issued, i.e., 192.168.0.# or 192.168.1.#, OR 10.0.0.# ?

Because if you do, you could try manually assigning an ip-address to your ethernet interface, and then see if you can ping your gateway or beyond.

Assuming that your gateway has 192.168.1.1 (very common that gateway uses 1, but not always), then you could try
this assigns the second address in the 192.168.0.# address range, and then, if your gateway uses the first, you could try pinging it:
If the pings return times in ms, then a connection has been established to the gateway.

If it gets that far, check if you can ping an external server, such as google's nameserver, 8.8.8.8

If that returns time in ms, then you've got internet, but you'd still need to make sure that there is a nameserver configured in /etc/resolv.conf, which expects at least one line to start with nameserver, followed by the ip address of a nameserver, so you could add "nameserver 8.8.8.8" to that file, and surf away.

It needs a password. It's Ethernet, but the modem requires a password for Ethernet too.

Network Manager does not understand that. Wicd does, but wicd was removed from Debian 11 and 12.

My brain was not thinking yesterday. I assume you are asking about 802.1x security? If you select settings for the wired connection in the NM applet the last tab is security. You can turn on 802.1x security and enter the authentication settings for your MODEM.

That field is grayed out for Ethernet.

The tab is grayed out? It works for me. I was playing around with a debian 10 system and broke network manager. I never figured it out but I was replacing it with 12 so I didn't care. The only package extra package I installed on 12 was for OpenVPN