Plex does not currently support remote access over IPv6 but it should work over IPv4. I’m not sure what is going on with your IPv4 connection though. You should be able to specify port forwards in your router (or request your ISP give you access to the router or have them do it for you since it is theirs).
If you want IPv6 remote access you can vote for it by clicking on the Vote button at the top of the page here: Ipv6 support for myplex
