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Hello , 

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hi,

in order to rediscover/re-add the FortiAP, one method would be to power cycle it, turn off and then back on and it should be rediscovered when it will be doing the DHCP request and it reaches the FGT and then you auth it again.

for the SSID, you must create it and assign/select it in a AP profile which you then assign to the FortiAPs, https://docs.fortinet.com/document/fortiap/7.6.4/fortiwifi-and-fortiap-configuration-guide/140799/creating-a-fortiap-profile 

Did you 'register & delete it' from a FortiGate?
If you like to manage it again, you can simply re-create it, either by resetting it and let the FG discover it*, og by manually adding the FortiAP via the serialnumber:
Wifi & Switchmanager: Managed APs: AP: Create New:

Enter the Serial and the model should be added and you can select an Operation Profile.

If it's not auto-authorized, do that.

* The FortiAP needs to be on a VLAN with 'Security Fabric Connection' enabled:

Network: Interfaces: fortilink: <vlan>: Administrative Access:

The AP's do not make a 'fortilink' on the switchport.