# "if the problem affects more than one device and happens at random times during the day"
it happens on all device at random occurence.
# "physical cable connection issue somewhere from the AP, through cable/wall drop/patch panel/switch/fgt."
to isolate the issue, i use a new cable connection, different port. tested working one without issue (previously used with other AP with no issue) same issue.
# "failing network switch/duplex/speed mismatch/network loop"
to isolate the issue, i directly connect it to FGT. internal 3 and created new subnet and created IPv4 policy to allow internet access. same issue.
# "rogue DHCP server (e.g. router) plugged into the network somewhere or devices assigned static IPs."
i created an IPv4 Policy to deny all traffic going to inter3 port where the wifi is connected and allow internet traffic to internal3 port.
# "DHCP IP lease exhaustion"
under internal3 port where the AP is now connected, has a new IP Pool range. and less than 10 device is connected.
#"To help with troubleshooting, enable Device Detection on the internal/SSID interfaces and go into the "User & Devices->Device Inventory" page - the online device info should in theory match the devices assigned IPs by the DHCP server (service). "
=Device detection is enabled and on Device inventory and DHCP monitor it is showing the device online with correct IP.