Wireless Devices 'Dropping Off' the Network

Setup:
Hardware: SG135
Software: UTM v9.705-3
APs: 2x AP55s
Non-Standard Config: AP Country set to US (not to my actual location), for hardware compatibility reasons

Problem:
For at least the last year, wireless devices on my network have been intermittently 'dropping off' the network.  This happens to single devices at a time, but multiple devices of different types from different vendors and different OSs are affected (ie; phones, tablets, laptops, TVs, set top boxes etc...)

What I mean by 'dropping off' is:

• The device shows as connected to WIFI
• The WIFI signal indicator on the device often shows with an '!' and says 'Connected, but Internet not available'
• It is not possible to ping another device on the LAN or elsewhere
• On the firewall, the status of the device shows as 'Online', with good signal strength & often good speed (but often this subsequently drops down to 24mbit)
• According to the firewall log, a device which has 'dropped off' still registers as sending packets to the net
• Physically moving the device to try to get a 'better' signal has no affect
• Wireless logs sometimes show the last entry for the device as 'disassociated due to inactivity', but not always; sometimes the device has not disconnected according to the logs
• The device can stay in this state for many hours and generally you must 'disconnect and reconnect' from the wireless to fix it, which almost always works.

Troubleshooting Steps Taken:

• I am unable to pinpoint the exact time that this issue started, however there have been no hardware changes and was not a problem for many years prior
• I have been unable to find any evidence of consistent issues in the wireless logs which may explain this or point to a common problem
• There are no related issues shown in other places such as the Intrusion Protection or Advanced Threat Protection logs or anywhere else I can find
• I have observed no patterns to this issue and it appears to affect virtually all wireless devices intermittently and randomly; sometimes not affecting a device for weeks at a time and other times affecting it multiple times within an hour
• I have tried disabling 'Fast Transition' and 'U-APSD', both of which I have observed to cause problems in some environments, but no change was observed
• Yes, I've tried turning it off and on again :)

So, I'm stumped.

Any suggestions / ideas / input would be greatly appreciated!