The hostname (or jail name) is not identical to the jail ID; iocage adds a prefix "ioc-" to make up the jail ID, so if your jail is called plex, you need to do jexec ioc-plex
Also, if there's a dot in the name, it will be replaced with an underscore.
Run jls -N
to see the actual jail IDs. Note that only jails that are "up" will be listed.