In a very basic diagram, this is the hardware configuration and network layout you have described. Is this correct?
It seems that the Ubuntu server is reaching out and getting NTP updates from its source. As long as Port 123 is open and the service is automatically restarted upon server reboot it appears to be functioning as expected.
The 192.168.2.101 IP address can be used by any/all systems within your LAN as NTP source. The camera's setting is to update its time every one minute. Once tested and validated, you could change this to once every 15 minutes; but that's a setting of your choosing.
Also, as the camera is on the same network address (192.168.2.0/24) you do NOT need to provide DNS entries on the camera. (i would recommend removing these entries as to prevent the camera from phoning home)
Do you have DNS running on the Ubuntu server? If not, do not use its address as a DNS server.
At the bottom of the GENERAL/Date & Time TAB you should be able to update (i.e. test/validate) the camera's date/time setting by clicking on the REFRESH button.
EDIT:
I presumed that the Ubuntu's IP address is either statically assigned or reserved in DHCP. If not, that needs to be corrected.