Recommendations on how to troubleshoot DHCP issues with adtran wireless and VLANS

I am having issues with adtran AP's and wireless DHCP.

I have two SSIDs, a native VLAN and a guest wireless network. The AP's are plugged into an HP switch with appropriate VLAN tagging

The issue I have is that when a client who was connected to the main network connects to the guest network, they retain their DHCP lease from the Windows server where DHCP is being handled by a fortigate firewall.

If I have them drop the connection on the phone for example and then drop the connection on the controller, they get the proper DHCP lease.