Steps to isolate AP/ZD/SZ/VSZ connectivity issues.
Network-Based
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Check the L3 Connectivity (Proper Interface)
Check the L4 Connectivity
Check the port :
(ZD - 1222, 1223 (LWAPP) , FTP)
(SZ - (mDNS)5353, 443, 91 (HTTP), SSH)
Latency - MTU
Administration Based
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Does the controller have enough license (Administration>License)
Is the AP model supported by controller(also the respective zone) firmware (Release Notes of each firmware/device)
Are AP and ZD/vSZ in same country code (AP - "get country code" ZD - GUI>Monitor>System)
Is AP approved by SZ - (GUI>Monitor>Access Points)
Staging Zone
Resources
Services are all online
Verify AP cert
lwapp2scg (enable/disable)
One of these steps definitely will help you isolate the issue.
Hope this helps.