We have a default "Client Isolation" option enabled for security purpose to isolate wi-fi clients connected through the same Ray AP ssid and the same network will never be able to see each other, to make all the clients reachable to each other you can disable this option by unchecking "Client isolation" as marked in below snapshots.
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Profiles --> Available profile --> Wlan
Wlan --> Wlan profile ( in this case "CP WLAN") --> Action menu --> View
WLAN profile ( CP WLAN) --> Advance --> Client Isolation ( untick this option to make all client see each other)
