NAS cannot access Internet

[spadekevin]

Hi,

My DS411J was working fine for 2 years, in LAN and remote access. After the router was changed by ISP, I still can access my NAS from Internet but the NAS cannot access Internet (No more Notifications, no access to Package Center, QuickConnect stopped working, etc...). I got messages such as: "Connection failed. Please check your network settings" or "Failed to retrieve external IP address. Please check your router's settings".
The router is SAGEMCOM F@st 3784b. I checked port forwarding rules in router, IP address, etc...
On router forum (French Bbox), expert said this router should be compatible, problem should be "something blocking in NAS" but I cannot guess what got wrong...
Any idea to help?

Please help.

I didn't find the right solution from the Internet.

References:-
https://forum.synology.com/enu/viewtopic.php?t=80316
Telematics Solution Video

Thanks!