Upgrading FreeBSD in a jail

I'm currently running FreeBSD 10.3 in a FreeNAS jail and would like to upgrade to 11.0

I tried this:-

freebsd-update -r 11.0-RELEASE upgrade

but was unable to get a public key from any of four servers.

Am I doing something wrong or are the servers to available?
Note that you cannot run a higher version jail on a lower version host. So running a 11.0 jail on a 10.x host is not possible. The other way around (a 10.x jail on a 11.0 host) is not a problem.
I'm not sure I understand.... I'm currently running a 10.3 jail on FreeNAS 9.10....

Would the 'fetching public key' failure error be due to the upgrade not being allowed?
FreeNAS 9.10 is based on FreeBSD 10.3. So you can only run, at most, 10.3 jails on there, you cannot run an 11.0 jail. The version of the jail can only be lower than the host version.
There has never been a FreeBSD 9.10 version. The last 9 version was 9.3, which was EoL'ed at the end of last year.