This issue has been happening for a while but it's at the point where it's driving my insane.

I have a Unifi network:

• UDM Pro.
• U6 Lite in 5ghz (8040Mhz) / 2ghz (2040Mhz). This has 2 SSIDs with a dedicated 'IOT' SSID that runs 2Ghz
• AP Lite (Wifi 4/5) downstairs running 20/40Mhz

2GHz devices seem to connect and work fine, but over time just seem to become unresponsive over time. This can take minutes to hours or days. Unifi controller thinks they are still connected, but any pings to them from other devices (i.e my Mac) timeout. 

If I go into Unifi and click 'Reconnect' they seem to come back alive for a bit before becoming unresponsive again. This seems to affect IOT type devices, like my Sonos, WiFi heat pump controller, Elgato Wifi light, Printer etc.

I'm not running any custom VLAN rules or anything, just a basic network. I've turned off most 'fancy' settings like Fast Roaming etc but nothing has fixed it. 

I might try remove the U6 Lite and see if the old AP (currently downstairs) runs any better, but I haven't really been able to narrow it down to an AP problem or a routing problem yet. Does anyone have any tips?