If the settings on your router are otherwise correct, please check the NTP version used by your NTP server.
Our firmware uses NTPv4, which is backwards compatible with NTPv3.
However, some NTPv3 server implementations, such as Windows 10 servers, only accept NTPv3 headers and time synchronization with such servers is not possible.
Please, contact your NTP server manufacturer to find out if this applies to you.
If your router is behind an external firewall and you are not using a local time server, UDP port 123 must be opened for the router for time synchronization.