Hub 5 - No internet after factory reset, modem mode not working properly

I've had my Hub 5 running in modem mode with a GLiNet router for ages without any problems. Went away for 3 weeks and halfway through noticed my GLiNet had lost its public IP. Could still get to it through GoodCloud though and Ring doorbell was still working so didn't panic.

Got back yesterday and foind it had set itself back to router mode somehow, didnt think much of it and would sort it in the morning. However,  woke up this morning to no internet at all.

I've factory reset the Hub 5 about 6 times now. The status page says it's got an internet connection but nothing I connect to it actually gets online. Tried my laptop, tablet, phone - all on ethernet and wifi. They connect to the Hub 5 fine but then just sit there with no actual internet.

Tried switching it back to modem mode loads of times. Green light comes on like it should but then I can't get to 192.168.100.1 on any of the ethernet ports and my GLiNet router just sits there getting no IP at all.

Virgin's website says no problems in my area (SS14). Waited 45 minutes on the phone this morning only to be told there's actually a network fault they haven't bothered updating their systems about. Fair enough if there is one, but the operator won't even consider that my Hub might be knackered until this mystery network issue gets sorted.

Thing is, I reckon the Hub 5 has actually died. In router mode it connects devices but gives them nothing, in modem mode it looks right but you can't actually talk to it. That's not normal behaviour is it?

Anyone else had a Hub 5 just give up like this? Wondering if something happened while we were away - power cut or dodgy firmware update maybe.

Also if anyone's got tips for getting Virgin to actually look at replacing faulty kit rather than fobbing you off with phantom network faults I'd be grateful. Been messing about with this for hours now.