Not an apex domain
The domain 2001-1960-0020-1000-0000-0000-0000-0000.0481e5d869583a5b6a5b11cc47f12a26.ip6.frontiernet.net is not the root of its zone.
Instead, try zone transfer on ip6.frontiernet.net .
Instead, try zone transfer on ip6.frontiernet.net .