Wired and wireless internet access both misbehaving on latitude e6230

I recently bought a few secondhand latitude e6230s, and I'm having serious issues with internet connection on one of them.

Initially, I thought it was probably some wireless-related issue (hardware or driver), but the problem persisted when trying a USB wireless adapter, and subsequently when using a wired connection.
This doesn't seem to be a problem with the network connection hardware itself, as on a wired connection I can copy data from a local machine at ~100Mbps

However, it doesn't seem to be *obviously* software-related either, as the issue persists after a clean operating system install, and even appears to surface during the OS install process itself. as there are delays and repeated retries when (for example) trying to choose and enter details for a Microsoft account in the WIn10 install process, and that has happened on multiple runs through a clean OS install.

The symptoms in Win10Pro (after a fresh OS install) are that internet connection on wireless and wired networking work to a very limited extent, such that win10 authentication happens (maybe needing a couple of tries), and it's sometimes possible to get search results in Edge, but actually getting to a found site very rarely succeeds, and  I've never managed to get to a Dell support website.
I Installed Chrome after a few attempts where the install stub couldn't download the program, but it doesn't work any better than Edge.
Attempts at windows updates or online search for driver updates from device manager have always failed, after a meaningful delay.

Looking at network activity in task manager shows some traffic when something is attempted, but often just a few spikes followed by silence, at least for processes which give up easily, and there's no rogue process hogging the internet connection.

The BIOS has been reset to defaults, just in case, but that makes no difference.
Windows memory test doesn't show any errors, nor do Intel processor diagnostics.

Apart from the networking issue, the machine seems to be working fine. Other PCs on the network, including the other two e6230s and my personal e6230 and netbooks via wireless, and wired desktops all work fine.
I've been managing PCs on and off for a few decades, but I don't have detailed knowledge of the Latitude, and I've run out of things to try, and I'm puzzled as to how the same misbehaviour is happening with inbuilt wireless, USB wireless and wired connections, as (though I may be wrong) that's hard for me to attribute to a common hardware issue (especially given that LAN networking works fine), but given that the problem seems to surface during a clean OS install, it's hard to blame it on software/malware either.
I have tried cloning a different e6230's HDD to the problem machine and changing product keys afterwards, but I get the same networking issue, which didn't affect the source machine.

I could just return the laptop, but it'd be nice to get it working if I could, or at least be able to point to something more specific as an issue.

Any suggestions for things to try, or to look for in windows log files or suchlike that might point to a cuplrit, or some hardware test tool which would examine networking?
Do bear in mind that because no internet connections work more than minimally, I can't get to Dell support website from the laptop to attempt firmware updates, driver updates etc, so anything along those lines would have to be possible via downloading on a different PC.