Leaving this here for those who get the IPv6 error on step #1.

  • Firefox: add invite.wtfnzb.pw to network.dns.ipv4OnlyDomains in about:config.
  • Chrome, Edge and forks: launch the browser with this command-line parameter.
    Code:
    --host-resolver-rules="MAP invite.wtfnzb.pw 104.21.25.115"
  • Others [universal solution]: add this to your hosts file.
    Code:
    104.21.25.115 invite.wtfnzb.pw

The main site works fine over IPv6, so you can undo the above fixes after registering.