Reconnecting to Unifi-AC after Hard Reset
Connecting to the Unifi AC after after a reset requires you to connect manually using LAN to the ERx or switch that the Unifi is connected to get back on the same network. From there, launch the Unifi Controller and it will take you back to the login page.
Log into the "devices tab" and click the "Adopt" option under the status field.
When adopting the AP to another IP use the steps listed in this link;
https://community.spiceworks.com/how_to/9692-manually-setting-the-controller-address-for-a-unifi-ap
Supplemental information;