Not a lot to work with but let's try...
Is the following enabled in the AP advanced tab..
- Maintain client sessions in event of poll failure
- Restart service in the absence of controller
https://community.extremenetworks.com/extreme/topics/maintaining_wireless_services_upon_hipath_contr...
What kind of authentication is used for the WLAN service.
Even if there is no wirless controller at the remote site why not put the controller behind a firewall with port forwarding for the AP tunnel and connect the APs via the internet to the HQ if VPN isn't possible (the only thing not working in this setup is the AP software upgrade function).