Virginmedia in the UK do not offer IPv6 I am a afraid, from what you have described it sounds like they are just passing the blame. There is also no setting for enable reflexive connectivity on the hub.
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While I am an IPv6 advocate lack of IP address space but also improved protocol making it often slightly faster the support guy at Discord is talking rubbish
Discord has not problems connecting using IPv4 as long at you have access to high UDP port (45000-60000 (UDP) for voice RTC) outbound from your machine or not blocking at the router (By default Virgin do not block on there Hub) or other device before. I use it no problem
Perhaps check your Machine does not have a Firewall in place blocking outbound (Some Anti-Virus products come with there own)