Where can I find the model-name and serial-number of my Zyxel device?

Find it on the Information Label (back label) of your device.

Example 1: NAS540

Example 2: VMG8324-B10A

Example 3: PLA5405

Find it on device’s front panel

Example 1: USG40W;
Example 2: NXC2500;
Example 3: GS2210-8

Find it on device GUI/Utility GUI

Example 1: NAS540

Example 2: USG60