@johnpoz I did go thorough port forward method. Initially i believed that setting the specific networks (172.x.x.x, 192.x.x.x, etc), in Virtual IPs, would be the only thing i need to do.
My first NTP server ran CHRONY but for some reason it was not able to provide time to the embedded linux devices. My Windows computer synced pretty fast.
Now, I switched to NTP daemon under Ubuntu. All embeded linux devices are syncing.
I also disabled the port forward and... the devices sync. Maybe it was because of the Ubuntu minimal server that i use.
The test device is not running firewall so i would pin point to the NTP server.
Guys, sorry for this but i am new in this area... Thank you for guiding and having patience with me. I'll test more tomorrow and if i can get away with networks defined in Virtual IPs, this is the best solution that i need.
Thank you, once again !
Best regards,
J.