Isolate the AP from the network on which the SmartZone is, also from DHCP if the goal is to use the APs default IP address. The initiate the FD procedure (Press and hold the reset button for 15 seconds). This should then put the AP into a factory default state. It might be that the AP is rejoining the SZ if you do not have it isolated.
The credentials for APs will have been set in the SmartZone. You can easily change these from the SZ GUI under the Zone for which the AP resides: