First please ensure that the modem router works fine which means you can get internet from it; then you can change another NTP server and give it a go: go to system tools-time settings to do that. 

Of course, my router can access internet, it's my wifi hotspot.

Putting other ntp servers (or ip address) is not functional...

I've googled the message and I've found this dead link (unfortunately not in google cache nor accessible via wayback machine) :

https://forum.tp-link.com/showthread.php?1844-Get-GMT-not-working

So, the summarize of this google result is :

7 janv. 2013 - Get GMT throws an error. ... All the TP-Link routers set date-time only from the WAN port, which is ... TP-Link Archer C7 v2 (WDS Bridge) ...

If someone can leads me to a newer version of this post, I would be very happy.

Tried to switch cables in order to have the tagged 'wan port' directly connected to my dsl modem, tried to get the router operating in 'adsl router mode' or in ' Wireless Router Mode', but no way...

Finally, ound another post for tp-link material (Archer C9) : in order to get time, I've to configure a static route because the router does not know 'how to exit my house' !

And it works...

Glad to hear that you solved your problem finally. And thanks for your feedback. 

We feed back this issue to our related engineer and they will work on it.