@timpotter you should enable your ipv6 firewall asap again. With IPv6 you don’t have any NAT anymore just
a firewall if you router has any. Otherwise every Client can be connected directly form the internet!!!.
So unless you have a firewall on your router or your Client you are basically complete open.
You also need to set your ipv6 address of your Plex Server not the router. As again there is no NAT that does translations.
@lawbaer First of all if your ipv6 address changes depends on your ISP.
They give a prefix. e.g. XXXX:XXXX:XXXX::/56. This prefix is announced to all your clients in the network and they build their unique addresses. with prefix: followed by “local part”.
Normally macox and windows (not sure if linux has privacy extension enabled by default) will have two ipv6 address. One is static in the case that is always prefix: local part. local part being mac address or hash of that. depends on System. But that local part never changes. so you should map always that address.
And of cause you need to open the localpart:portnumber in your firewall. This is complete separate from the portmapping settings in your router. Depending on your router if you can configure that.
The other Address is random and does change over time. That is so you don’t browse the web with always the same address so that tracking is not that easy. That why that’s called privacy extension.
Take away for everbody:
Ipv6 has no NAT so you need to make a direct connection to your Plex Server ipv6 address. Depending on your router you might have a good ipv6 firewall where you can open port to specific client address. But I have seen many where you can block just all ipv6 incomming traffic or disable it completely. Which sucks and should only be done when every Client in your network has a Client Firewall or doesn’t expose any services to the outsite. known and unknowns (which will be the even more fun part in the future with ipv6 fridges etc 
)
Hope this makes it a little bit more clear. Otherwise read up on ipv6 on other sites.
Don’t want to explain to much here.
