Having my DHCP server configured with an additional external DNS server did work - of course, as long as the Samba DC jail is down, AD domain access is unavailable, but at least my users are able to browse their Facebook and TikTok accounts 
As for the jails themselves, I’m not sure why they did not start on boot, even though I have the correct
jlmkr.py startup
command set, and this particular machine has been rebooted uneventfully a few times in the past…
I have put an additional mechanism in place to force start my jails manually a couple minutes after TrueNAS boots. Hopefully this will solve my immediate concern, that of losing administrative access to the server.