Strange internet connectivity issues

[Chevens]

Would appreciate any insights into the situation I find myself in.
I've a BT infinity 2 service, with a home hub 5. On Wednesday evening my connection died, BT acknowledged an outage in my area. When BT 'fixed' the issue my connectivity was dire from my PCs - both connected via Ethernet to my home hub. trying to run speedtest.net or other bandwidth benchmarks was difficult, often the website timed out before doing a test, on the few occasions a test started I was seeing between 3 - 10 mb/s down and 1 mb/s up (normally 60 down and 12 up). After a few days and resets of the home hub and PC nothing much has changed.
The strange thing - my TV is happily spewing out 4k streams from amazon/netflix without any hiccups (wifi). My PC happily streams amazon prime or netflix content without buffering, but youtube, and all other websites are almost unusable. Tested on multiple PCs. Mobile phone is also having issues web browsing, although strangely seems a bit more reliable than either PC.
Any ideas?!

Thanks

 

[Chevens]

As above, log into the hub (192.168.1.254 from memory) and check the sync speed. Also try a different speedtest server and even a different speed test site, sometimes ‘auto’ for location throws a curve ball/isn’t your best option as it just goes off ping.

Thanks, not sure where to find sync speed, DSL connection status shows

Line state: Connected
Connection time: 0 days, 00:21:17
Downstream: 50.43 Mbps
Upstream: 12.07 Mbps

This isn't accurate to my service right now - web is extremely slow and multiple refreshes are required to even load a page sometimes.
Also video streaming still working, but at lower bit rate.

Also keep getting the following error
Your connection is not private
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NET::ERR_CERT_COMMON_NAME_INVALID