Here's a post from another knowledgeable forum member:
On the Hub 3 a red light is the overheating indicator. First thing to try is a pinhole reset ( making sure you've got the eight digit admin password still printed on the base of the hub, because this resets the hub to factory settings. That can work perfectly, as the hub is a firmware controlled device. But if you try that, and the light is still red, then the hub needs replacing, either because it is overheating, or because it's overheating indicator is faulty (ie a safety issue whichever way).
Personally if I had a Hub 3 showing a red light, I wouldn't leave it powered on overnight, or if leaving the house for extended periods of time. That's probably a little bit OTT, but better safe than sorry.
So do a factory reset (60 seconds press of the pinhole button and leave to boot back up) and if that doesn't clear it request a new hub from VM - ring at 08:00 in the morning when lines are likely quieter.
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I know a bit about Wi-Fi, Telecoms, and TV as I used to do it for a living but I'm not perfect so don't beat me up... If you make things you make mistakes!