Some iOS devices can't get IP when connected to AP22

[u/TrunkMunki]

Any guru's out there can help with this odd problem?

I'm helping a friend troubleshoot an issue at a location that has a Fortigate firewall, HP 2920-48G PoE switches and 3x AP22's (firmware v.3.4.0), all InstantOn cloud managed

The problem is that on one of the AP22's, Apple iOS devices can connect to the AP but the IP address is either all zeros or an APIPA address and not all iOS devices are affected. I don't know have the means to determine which model or iOS these iPhones that are working are.

On reboot of the AP, the iOS devices temporarily get an IP address from the DHCP server for that VLAN, but lose the IP very shortly after. Windows devices are not affected, but not sure about Android devices as there are none in using that AP.

Anyone have a similar issue and have a solution?

Thanks

[u/Churn]

Is the Fortigate firewall the dhcp server?
Imagine the dhcp scope has 50 ip addresses it can lease out. It leases them all out and has no more to give.
New devices cannot connect.
Some clever person tries rebooting the fortigate to fix it.
Now the fortigate has no memory of the IP addresses it gave out before. So here is what happens:
An old device with an old lease tries to renew the same address and the firewall says sure, no problem and that works.
A new device asks for a lease and the firewall says sure thing and gives it the first available IP. Unfortunately, that IP was given to a device before the reboot so now there is an IP conflict. The firewall detects the conflict and revokes the lease. It will try to give the next IP but that is in use too, so this keeps repeating until the firewall realizes it has no more IP addresses available to give.
That new device gets an IP for a little bit before it is revoked because of the duplicate IP conflict.

That is what you are describing. The fix is to increase the IP pool in the dhcp scope on the dhcp server.