Sorry to bring up an old thread, but I'm starting to have this issue now.
IPv6 enabled, can't access PS Store, PS Plus, friends list, even movies I've bought on the PS Store.
IPv6 Disabled in both LAN and WAN, Reboot the PS5, and can access all 4.
I tired the MTU and setting my secondary dns to 1.1.1.1, but still has the issues with IPv6 on