Log into your Fortigate with SSH and enter the vdom context you are using then edit the WAN interface:
Assumtion
wan1
is for pppoe portinternal
internal switchvlan30
guest vlan
config system interface
Log into your Fortigate with SSH and enter the vdom context you are using then edit the WAN interface:
Assumtion
wan1
is for pppoe portinternal
internal switchvlan30
guest vlanconfig system interface