Thanks Connor.
Yes, I’m on the latest firmware of the router but I believe an update is currently rolling out for the device.
I am using a Three branded Zyxel NR5103E router. No VPN’s/Proxies or firewalls in place.
I did migrate the core from a NUC, restoring from a backup.
I’ve got another broadband network in the house (Virgin Media Fibre) so I can switch to that for further testing, but I don’t want to run on that long term as its frankly terribly unreliable.
I’d also point you to another Support thread with the same/similar symptoms. We both seem to be running on Three 5G internet: