Hi,
The following points should help :
1. Subscription key should be added on your Central Account.
2. Adding devices to Central:
Adding Devices to Central After you successfully log in to Central, a welcome message is displayed in the Central UI.
a)To bind devices to your subscription, click Manage Your Devices. The Device Provisioning page is displayed. To view the subscription key details before binding devices, click Subscription Keys under Global Settings > Devices & Subscriptions.
b) Central supports zero touch provisioning of the devices. It automatically retrieves the devices associated with your subscription and Central subscription. To synchronize the devices from the inventory, click Sync Now.
c) If the retrieval of devices is not complete or successful due to process errors, you can manually add the devices.
d) Central allows you to import devices using your Aruba Activate user credentials, the MAC address and cloud activation key of a device, or the MAC address and Serial Number of a device. You can specify the method for importing devices when adding a device. For users with the evaluation subscription, the devices are not automatically synchronized. Therefore, the users must manually add the devices
3. IAP side:
a) Make sure the clock on IAP is correct.
b) They are assigned a DNS server which will help in resolving the different URl's which the IAP tries to communicate with.
I have attached the Central UG which clearly outlines the steps needed for adding IAP's on Central.