Apologies for not actioning every test in the order requested. I really do appreciate the time everyone has spent on this and I think that we have made genuine progress.
Been able to spend a decent block of time on this now at my laptop, recent messages have tended to be repeating information or researching, so here is a summary and test results.
The problem: internet access lost on laptop triggered by file download while retaining connection to router via WiFi, cable, or tethered with smart phone. Smart phone does not have these issues at the same time or on its own. Speed tests on both devices report serious bufferbloat. Router = Plusnet Hub Zero AKA Sagemcom 2704N
1. Reconnecting WiFi on the laptop used to resolve the problem but that ceased to work a few days ago.
2. Rebooting the router does not fix the problem.
3. Logging out and back in again on the laptop does not fix the problem
4. Restarting the laptop does fix the problem.
5. Leaving the laptop in the error state with no particular upload or download for 20 minutes does not fix the problem.
6. The problem still occurs when connecting the laptop to the router by ethernet cable.
7. Problem still occurs when accessing the router via USB tether with mobile phone. Meanwhile the phone works as normal.
8. Problem does not occur when accessing the internet via WiFi tether to mobile phone connected to 4G network.
9. Speed tests on broadband report good overall connection speed on upload and download but ping often peaks between 1 and 3 seconds.
10. Problem still occurs when WiFi connected to router and altering available bandwidth setting on Killer WiFI Control Centre.
11. Killer WiFi Control Centre does not report any dropped packets but documentation is poor so I'm not sure I trust it because I cannot dig into how it would pick up dropped packets when presumably there should be lots due to bufferbloat?
12. WiFi device driver updated a few weeks ago as part of Killer Wifi Control Centre installation. Not sure about BIOS, kind of thing that I would do but recommendations were mixed and somewhat cynical in the manufacturer's forum. I don't have a distinct memory of doing it (had the laptop for a couple of months now and I was updating and setting up a few things when it arrived). Forum said don't risk it unless fixing something specific. This could be such a situation so I will look into it.
13. Laptop manufacturer's forum does not appear to have any history of similar issues but no response so far to the bufferbloat suggestion.
My thinking so far is as follows: The Plusnet router has a poor reputation and/or my one is faulty, resulting in a bufferbloat situation. However, the phone can still access the internet albeit with very high pings. So for some reason the laptop and the router are not handling the flow of traffic well enough between themselves and one or both buffers are overflowing. Rebooting evidence suggests it's the laptop buffers (and possibly the phone not being impacted is more evidence, does each connected device get its own fenced off bit of buffer in the router?) With a well performing router the laptop does not lose internet e.g. tethered to the phone on 4G (I forgot to run speed test so will get that done soon.) This implies that the laptop or its software is not faulty as such, but might have a weakness when interfacing with a poor router. Perhaps somewhere on the router or laptop there is a setting that can mitigate these issues. Not too keen on buying a new router when Plusnet have provided one with such a poor reputation - guilty until proven innocent? Or I need more evidence before returning the laptop (in which case a new router would be a waste of money, would performance increase that much in a two device household on basic broadband?)
