Has the FortiExtender been updated to the latest firmware? it likely won’t connect otherwise. After upgrading to the latest FortiExtender firmware, be sure to perform factory reset and then apply the configuration documented here.
Is the FortiExtender connected to the same FortiCare account as the FortiSASE instance?
Is the FortiExtender connecting to the leader? There is currently a known issue on FortiOS where a device will only get detected if it connects to the leader FortiGate. If you don't see the FortiExtender in the FortiSASE portal to authorize, open a FortiCare ticket to determine the hostname of the leader FortiGate for your instance.
Connect your Internet connection to port 1, and Local LAN to ports 4-5.
Note: it takes a few minutes to connect FortiExtender to SASE, after which it will take over DHCP and serve as your default gateway. Until then, traffic will traverse your local internet connection