Check the following based on the knowledge base:
1. DDNS / external access setup
- In DSM, go to Control Panel > External Access > DDNS to register or verify a DDNS hostname.
- A DDNS hostname can point to the IP address of your Synology NAS.
2. QuickConnect
- QuickConnect lets client apps connect to your NAS via the Internet without port forwarding.
- Go to Control Panel > QuickConnect to manage it.
3. Port forwarding
- If you want direct access from the Internet, configure port forwarding in Control Panel > External Access > Router Configuration.
- Port forwarding redirects traffic from your router to the NAS’s IP address/port combination.
4. Internet / network requirements
- The Synology NAS must be able to connect to the Internet.
- For initial setup with Web Assistant, the NAS and your computer must be on the same local network.
5. If remote access suddenly stopped working
- A community report says that after a router swap, remote access issues may be related to the router, DDNS update, or port forwarding setup.
- The thread also mentions that in one case the issue was linked to a router firmware bug and bandwidth control.
6. Protocol-specific access
- If you are trying to access shares using Windows CIFS from outside, one community post says it may be blocked by the router and/or ISP, and suggests using VPN Server and connecting via VPN instead.
If you want, I can also summarize the exact DSM menu paths for DDNS, QuickConnect, and port forwarding from the fragments.
Sources used: [2], [3], [8], [11], [13]