You might need to reset it once again, but this time hold the button for 30-40 seconds (just tob e sure):
Using the Mode Button
Using the Mode button (see Figure 6<>) you can:
* Reset the AP to the default factory-shipped configuration.
* Clear the AP internal storage, including all configuration files and the regulatory domain configuration.
To use the mode button, press, and keep pressed, the mode button on the access point during the boot cycle. Wait until the status LED changes to Amber. During this, the AP console shows a seconds counter, counting the number of seconds the mode button is pressed. Then:
* To reset the AP to the default factory-shipped configuration, keep the mode button pressed for less than 20 seconds. The AP configuration files are cleared.
This resets all configuration settings to factory defaults, including passwords, encryption keys, the IP address, and the SSID. However, the regulatory domain provisioning is not reset.
* To clear the AP internal storage, including all configuration files and the regulatory domain configuration, keep the mode button pressed for more than 20 seconds, but less than 60 seconds.
The AP status LED changes from Amber to Red, and all the files in the AP storage directory are cleared.
If you keep the mode button pressed for more than 60 seconds, the mode button is assumed faulty and no changes are made.
https://www.cisco.com/c/en/us/td/docs/wireless/access_point/1815/quick/guide/ap1815igetstart.html#19187
