Somehow sounds more like a local network issue rather than something wrong with the equipment if it's been fine for 6 months.What's weird is it worked so well for the last 6 months. No problems, multiple devices connected streaming video, audio web traffic etc.
Then it suddenly starts dropping out for 5 minutes at a time, usually at the same time like exactly on the hour (but has been inconsistent here too). I've disconnected various things (smart tv, smart energy meter, google home mini etc.. but no manner of disconnecting, resetting, rebooting, factory resetting has helped so far.
Going to try this MTU thing, and shoving the SIM in a phone. Thanks.
You're definitely connective through IPv4 only now?