Nebula device offline possible reasons

There are possibilities lead Nebula devices offline on Nebula Control Center.

[First]
You may not plug a power cord into your Nebula device(s).

[Second]
Your device(s) may have a hardware issue or it's faulty.

[Third]
Your device(s) might have the following network issues:

1. The device(s) does not get an usable IP address.
2. The device(s) cannot reach TCP port 4335/6667.
3. The device(s) cannot synchronize Network Time Protocol(NTP) via UDP port 123.
4. The device(s) DNS cannot resolve d.nebula.zyxel.com.

Use the links to verify if your network allows port 4335 or 6667.
Test port 4335
Test port 6667