In my experience, a good quality ethernet cable (of the right spec, obviously) is essential to achieve and maintain advertised speeds.

It is also good practise to keep a couple of unused ones handy just in case.
Those tiny/crappy plastic tabs thingies to keep them in place are a PITA.

Best,

A.

Maybe a reason for differences on your system?

my son is still connected via the wifi

A friend visited for a week. My connection wired, his wifi. The lost wired connection was from bandwidth. Possible explanation?

when you have the time, review of Michael Horowitz works/websites is sometimes educational and always thought-provoking.

ttps://www.michaelhorowitz.com/

ttps://www.michaelhorowitz.com/second.router.for.wfh.php

regarding cabling, connectors, and physical ports, it is good to establish their correct and complete functioning and connectivity

i have a second-hand laptop that has a damaged ethernet port(sold to me that way, unbeknownst to me...sigh) and it works properly if you jiggle it in a certain way a bit(the poor connection shows as a 100Mbps and after some jiggle it will usually become correct at 1000Mbps).

also i grabbed a random webpage as an example:
ttps://forums.tomshardware.com/threads/getting-10-100-instead-of-1000-mbps-network-speeds.1710132/

My Virgin-supplied cable-router phones home on every startup to check if there is a firmware update available. If so it gets & installs it. That will lead to several router reboots. Even if no update there will be at least one reboot.

Do you happen to have an eero router? Those things are notorious for this. In my experience, on Windows 10 the problem is very bad. I'll click on the Connect button, and it will connect after a considerable delay. But then, very often, it will disconnect immediately or within a minute or so. The problem is bad enough that when I run Windows Update I plug an Ethernet cable into the laptop and don't even bother with Wi-Fi. So far, on Devuan the problem has been nonexistent for me; however, I don't use Devuan with Wi-Fi very often, so the problem may still be there but with much less severity compared to Windows.

I did see references in user.log sys.log and dmesg, but mostly early initialisation.

user.log tells me cron is complaining about not have mta for internal mail (to nobody but me) but doesn't explain the drop outs.

I think if I were to search further, I would check the logs inside the modem/router, But I am not suppose to access the modem, It's not mine.

I'd be interested to see what may cause the drop-outs.

Is there any kind of log I can check for this? Or a message in the network manager? I never had this problem before.