Re: GUI Access unavailable on fortigate 30e
First, you need to find out how those lan ports are split to make the "DMZ". Is that lan4 port or other port?
It's under "config sys virtual-switch". "show" would show all ports in hard switch "lan". If it's separated you wouldn't see the port.
Then check what IP is configured on the port under "config sys int". Again "show" would show all config on all ports.
And then finally, find the IP configured as a gateway under "config sys dhcp server". Those are grouped with "edit 1", "edit 2", and so on. You probably broke the section serving the port subnet.
You should just search on the internet like "fortigate cli dhcp server configuration" then you can find a bunch of articles mostly inside of Fortinet's document library, which you can learn how DHCP server should be configured.