Hi Friend,
East way is let it join controller first and then you can reset the configuration of AP from controller GUI.
If that is not possible for you for any reason you can console into AP and issue a command "dir flash" and you will see 2 files name "env_vars" and "private" name file. Delete it from flash running a command "delete flash:env_vars" and same command to delete second file and just reload the AP and it will delete old configuration regarding controller ip address.
HTH
Ankur
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