The first time a jail is created (in a new or existing pool that has no existing iocage dataset), the IPv4 Default Router field is editable in Advanced settings. This field is not editable in subsequent created jails, even if original jail is deleted, unless the iocage dataset is deleted, in which case it all starts over.
Steps to reproduce:
Storage -> Pools -> create pool (if not already done)
Jails -> Add -> name/release -> Next -> IPv4 Router is NOT editable
Jails -> Add -> Advanced -> IPv4 Default Router is editable
2nd Jail and all subsequent jails:
Jails -> Add -> name/release -> Next -> IPv4 Default Router is NOT editable
Jails -> Add -> Advanced -> IPv4 Default Router is NOT editable
if the iocage dataset is deleted, the 1st Jail / 2nd Jail ... scenario can be repeated.
Also, FreeNAS version 11.2U5 allows selecting 11.3-RELEASE to create a jail, but fatally errors out when trying to use the jail. The iocage RELEASE version offered for fetching should be = to only the current FreeNAS release version.