Diagnosis
That alert means 'Client Isolation' or 'AP Isolation' is turned on. It's a security feature that prevents devices on your WiFi from seeing each other—great for a public hotspot, but bad at home if you want your printer, smart speakers, and phone to communicate.
How to Fix It
You need to turn this setting off in the app. 1) Open the AmpliFi app and tap on your Alien. 2) Go to 'Settings' (the gear icon). 3) Tap on 'Wireless'. 4) Look for 'Client Isolation' or 'AP Isolation'. 5) Make sure the switch is turned OFF (gray, not blue). The change takes effect immediately. Those alerts should stop and your devices will be able to see each other again.
If That Didn't Work
If the setting was already off, you might have created a separate 'Guest' network. Devices on a Guest network are isolated from the main network by design. Check your 'Networks' list in the app and make sure your problem devices are all connected to your primary network name (SSID).
Heads Up
This setting is sometimes accidentally turned on during a firmware update or if someone was tinkering with advanced options. It's harmless security-wise to have it off on a trusted home network.