How can I ensure that my Zyxel device connects to the Nebula control center online?

Zyxel_JonasTan

To ensure stable connectivity between your Zyxel device and Nebula, you need to verify the following criteria:

1. The device(s) do get a usable IP address.
2. The device(s) can reach TCP port 4335/6667.
3. The device(s) can synchronize Network Time Protocol(NTP) via UDP port 123.
4. The device(s) DNS can resolve d.nebula.zyxel.com.

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

Please visit this link for more detailed information.