I recently set-up a smart switch which I directly send commands through an app on my phone via its fixed IP address. Prior to this I never had a device on wifi trying to talk directly to another device on wifi, so this problem may be longstanding, but I have just recently run into it. Periodically (10 days or so) I stop being able to connect to other wifi clients from my phone. I can't force it to happen yet, so I have not debugged all possible connection scenarios, however it is notable that my phone is (typically) on 5GHz, and I could ping other clients on 5GHz (as well as all wired clients on my LAN), however I could not reach any 2.4GHz clients. While this happens I have no trouble connecting to 2.4GHz clients from my desktop(s) which are wired. I updated the firmware the last time this happened, but the problem is persisting. Previously, re-booting the 660HD solved the issue, and this most recent time, I toggled off the 5GHz to force my phone to 2.4GHz to see if 2.4 clients could talk to eachother, but it would seem the simple act of toggling the 5GHz off then on solved the problem similar to rebooting did a week ago. It may seem I will need to wait another week in order to possibly test another 5GHz client before making any settings changes to my AP, but I wanted to reach out to see if this was a known issue. Both frequencies are broadcasting the same SSID and use the same password/authentication, and most of the devices on my LAN have fixed IP assignments from my pfsense router.