FortiGate Firewall suddenly could not get DHCP IP

One day, FortiGate Firewall (FortiGate) suddenly could not get DHCP IP on one WAN line.

Tried to disconnect & reconnect it. 

Not work.

Immediately changed the routing via the second WAN line (Fixed IP).

Called HKBN (📞121 238) to test.

Confirmed that would be FortiGate's question.

Scheduled a day to reboot it.

Rebooted, then back to normal.

First time to get this question. 🤨

Update