It sounds more like what you need is a bridge via a software switch, instead of transparent mode.
-Remove *all* references to the wifi and lan interfaces in the firewall configuration: routes, DHCP scopes, policies, etc.
-Create New under System > Network > Interfaces, and choose Software Switch as the type
-Select wifi and lan (or their respective names) as the two members, and re-define the firewall's address, DHCP scope, etc.
-Re-craft your firewall policies and any other objects that were associated with wifi or lan previously
Am I mis-interpreting your question, and are you trying to access a LAN or DHCP server beyond the FortiGate in your internal network?