That BQM and the high upload/download data ratio looks like it supports my hypothesis,
that excessive upload latency is causing packet loss, causing packet re-transmission, causing connection to collapse,
and potentially the frequent packet loss is breaking the SSL handshake protocol, that manifests in the errors you see.
If your bufferbloat test "Upload Active Latency" is greater than +30ms, then somehow you are going to need to implement QoS, to prioritise and regulate the rate of traffic being sent from your network before it hits your modem.
What would make it deteriorate to this extent in the last 2 weeks? Line fault or something else?
The TP-Link router is a recent thing - I only added this because the second Openreach engineer recommended getting rid of Powerline converters. Hub one had been working reasonably well for the last 20 months and before that John Lewis broadband was OK.
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