Tony Seaford wrote

It can be solved if we create a static route rule on the router.

@Carl

TP-Link should fix not only this specific issue envolving NTP, but all other ones related to the WAN port not being used to connect to the Internet (that is, when Internet signal arrives through any of the router's Ethernet ports).

For example:

• Guest wi-fi networks allow connections but don't provide Internet navigation unless you allow guests to access our network.
• Internet connection status is misleading (it says there's no Internet connection available).
• "Internet Connection Type" cannot be configured to avoid the misleading Internet status.
• Still regarding NTP, scheduled reboots can't be configured when NTP is not enabled (unless we set up a static route to make it work - which is not an ideal solution).
   

Many of us use TP-Link routers as a second router in the same LAN, connected to an Internet gateway (e.g., a modem/router) via Ethernet cable, mainly because they have much more features then the ones provided by ISPs. It doesn't make sense that several of these features don't work just because the WAN port is disconnected...