Set the NAS to a fixed IP on your LAN. The knowledge base says this is the recommended approach when you want the NAS address not to change.
- Use an IP in the same subnet as the rest of your network.
- If your router uses DHCP, you can either:
- assign a fixed/static address on the router for the NAS, or
- set the NAS itself to a static IP.
Synology’s DSM path is:
Control Panel > Network > Network Interface > select the interface > Edit
Then set the IPv4 connection type to Static IP. If you want DSM to go back to automatic addressing later, choose Get network configuration automatically (DHCP).
For remote access from outside your network, the fragments say the key point is not a static external IP, but DDNS. Internally, the NAS should use a static IP so router forwarding doesn’t break.
Sources: [2], [3], [12]