Try browsing to https://plex.tv/api/resources?X-Plex-Token=your_plex_token_here&includeIPv6=1 to see if your server’s IPv6 address is being advertised to Plex’s servers; mine is. Work is being done, and my understanding is that with the correct setup, fully-IPv6 remote access actually works with some clients (mostly Apple, I believe).
By correct setup, I mean allowing access to your public network on TCP port 32400.
There’s more information here:
[Edit]
You can find your Plex authentication token using the Information in this article:
https://support.plex.tv/articles/204059436-finding-an-authentication-token-x-plex-token/
