@Fjordestan 

Hi,

Newer router models from TP-Link offer a dedicated "Access Point" mode and that would allow you to set the Default Gateway to be on the LAN interface.

Switching between "Router" mode and "Access Point" mode can usually be done via an "Operation Mode" menu, provided it exists.

On the older routers that don't have this feature the Default Gateway of the router itself is always via their WAN interface.

Are you just trying to get the TL-WDR3600 to synchronize its date and time with an NTP server on the Internet or are there other reasons?
 

  @woozle 

Date/Time synchronization would be one reason.  The other reason is for diagnostics (ping) to remotely check on connection problems.  This is 1 of 2 routers that I can't remotely diagnose internet connection problems.  I can access them via LAN, but I can't tell if internet is working without physical access.  I always assume that if I can talk to the router, then it must have internet, but I have constant complaints from tenants that their internet isn't working.  The ability to ping an external IP would do wonders.  New Access Points are not in the budget yet, so I was hoping there was a workaround.