In the beginning I had the same problem.
I don't really know if this is the most correct way to do it but it works. There's how:
I have been using a NTP server at address 192.53.103.103, so I put this address at the router interface, under IP Config/Miscellaneous/NTP Server. Then, under IP Config/Route/Static Route List I added a new route as follows:
Network/Host IP : 192.53.103.103 (this is my case; put the address of your preferred server here)
Netmask : 255.255.255.255
Gateway : 192.168.1.254 (this is my case, my 500g is connected to a ADSL modem router; put the address of your gateway here)
Metric : 0 (I am not sure about this one, but it works like this)
Interface : LAN (once again, this is my case. The ADSL modem router is connected to one of the 500g's LAN ports. If you use the WAN port for access to the world, put WAN here).
And that's it. I works.
Regards
Miguel