Welcome to my little problem 
I’m running a TrueNAS Core system for some years now and want to setup a second system for replication as backup for the main system (but that’s another story).
Both systems are “offline”. The main system has no default gateway set so it cannot reach the internet but is fully functional on the local network.
The backup system is NOT connected to the same network, it’s directly connected to the main system via a second NIC. The systems can reach each other (ping).
To avoid possible problems in the future I want the backup system to reach the NTP server which is the router.
My first attempt was to set static routes in the backup system and in the router with the main system as gateway. This was too easy so it doesn’t work.
Then I added a bridge to the network interfaces on the main system with both physical interfaces as members and it still doesn’t work.
Do I need to start a service or configure something else to get this running?
Alternatively: Is there a way to setup a NTP server on the main system so the backup system does not need to reach the router?
Yes, I could connect the backup system to the same network and everything would work but for testing purposes I want to set it up like described above so the backup system is only reachable from the main system and vice versa.