Another Windows migrant here. I can’t get my ethernet to work but wifi works OK. I am almost certain that when I installed Debian Trixie with KDE Plasma a few weeks ago, ethernet worked but it stopped a day or so later. Info Centre reports:

2: enp0s25: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN group default qlen 1000
link/ether 54:ee:75:52:01:23 brd ff:ff:ff:ff:ff:ff
altname enx54ee75520123
3: enx0050b6c0f7f3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 00:50:b6:c0:f7:f3 brd ff:ff:ff:ff:ff:ff
inet 192.168.1.92/24 brd 192.168.1.255 scope global dynamic noprefixroute enx0050b6c0f7f3
valid_lft 3419sec preferred_lft 2969sec
inet6 fe80::8437:d694:3204:62ff/64 scope link
valid_lft forever preferred_lft forever

I deleted the wired connection in System Settings | Wi-Fi & Networking and it was recreated which probably suggests the ethernet connection is detected even if the fields there are all blank. Also, the internet traffic plasmoid shows enx0050b6c0f7f3 with around 1/5 of the cumulative traffic of wifi.

I tried the obvious things, just in case. I disabled the firewall, restarted the router, deleted the wired connection, played with settings in Wi-Fi & Networking and tried dhcpcd.

$ sudo dhcpcd 
main: control_open: Connection refused 
dhcpcd-10.1.0 starting 
dev: loaded udev 
DUID 00:01:00:01:30:54:2e:d5:00:50:b6:c0:f7:f3 
wlp4s0: connected to Access Point: glocal 
enp0s25: waiting for carrier 
enx0050b6c0f7f3: IAID b6:c0:f7:f3 
wlp4s0: IAID 86:9b:42:5e 
enx0050b6c0f7f3: soliciting an IPv6 router 
wlp4s0: soliciting an IPv6 router 
wlp4s0: rebinding lease of 192.168.1.122 
wlp4s0: probing address 192.168.1.122/24 
enx0050b6c0f7f3: rebinding lease of 192.168.1.216 
enx0050b6c0f7f3: leased 192.168.1.216 for 3600 seconds 
enx0050b6c0f7f3: adding route to 192.168.1.0/24 
enx0050b6c0f7f3: adding default route via 192.168.1.254

and sudo systemctl status NetworkManager.service returns

●NetworkManager.service - Network Manager
Loaded: loaded (/usr/lib/systemd/system/NetworkManager.service;enabled; preset: enabled) 
Active:active (running)since Sun 2025-10-12 23:59:31 BST; 47min ago
Invocation: a3faea14d3dc48e29a2e2d27750ca082
  Docs: man:NetworkManager(8)
  Main PID: 98676 (NetworkManager)
 Tasks: 4 (limit: 9149)
Memory: 6.3M (peak: 7.1M)
   CPU: 2.457s
CGroup: /system.slice/NetworkManager.service
└─98676 /usr/sbin/NetworkManager --no-daemon

Oct 13 00:03:10 tpkde NetworkManager[98676]: <info>  [1760310190.8454] dhcp4 (wlp4s0): activation: beginning transaction (timeout in 45 seconds) 
Oct 13 00:03:10 tpkde NetworkManager[98676]: <info>  [1760310190.8623] dhcp4 (wlp4s0): state changed new lease, address=192.168.1.85, acd pending 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.0217] dhcp4 (wlp4s0): state changed new lease, address=192.168.1.85 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.0237] policy: set 'glocal' (wlp4s0) as default for IPv4 routing and DNS 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.0440] device (wlp4s0): state change: ip-config -> ip-check (reason 'none', managed-type: 'full') 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.0839] device (wlp4s0): state change: ip-check -> secondaries (reason 'none', managed-type: 'full') 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.0841] device (wlp4s0): state change: secondaries -> activated (reason 'none', managed-type: 'full') 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.0855] device (wlp4s0): Activation: successful, device activated. 
Oct 13 00:03:11 tpkde NetworkManager[98676]: <info>  [1760310191.1033] audit: op="statistics" interface="wlp4s0" ifindex=4 args="2000" pid=1511 uid=1000 result="succe> 
Oct 13 00:33:10 tpkde NetworkManager[98676]: <info>  [1760311990.8671] dhcp4 (wlp4s0): state changed new lease, address=192.168.1.85

Not sure if this is relevant, but DCHP is handled by pi.hole on a Raspberry Pi. This has been working serving multiple devices for a long time without issues. Also, this is temporarily a dual boot Windows/Linux setup. When I log out and into Windows, everything works as ever.

After several days trying, I ran out of ideas. Can someone help please.

EDIT: SOLVED! In case it helps others, reading https://wiki.debian.org/NetworkManager closely, I ran nmcli device which showed that specific ethernet interface as ‘unmanaged’. I am not sure why. Then, I followed the instructions below:

If you want NetworkManager to handle interfaces that are enabled in /etc/network/interfaces:

Set managed=true in a drop-in file in /etc/NetworkManager/NetworkManager.conf.d/ or directly in /etc/NetworkManager/NetworkManager.conf.

Debian documentation could be more accessible, but it is invaluable. Thanks all for your help.