1. Enable “Connectivity Uplink Monitor & Wireless Uplink”

In the Classic Interface go to: Settings > Site > under Services section, next to “Uplink Connectivity Monitor” put check mark in “Enable Wireless uplink”

Maybe in new interface go to: Settings > WiFi > AP Site Settings – Enable “Wireless Meshing”

2. Now you can connect the new AP to the wired network or you can just power up the new AP and adopt it.

3. Enable Mesh on APs, both the AP that will not be wired and the AP that will be wired

Select AP > Settings > Enable Messing

4. Disconnect AP that will not be wired to the network. This is the Downlink device that will connect to an Uplink AP on the network. It should take under a minute for the new AP to connect wirelessly.

5. On the devices list, under the “Connection” column it will list to which AP the new Meshed AP connected. To see more detail, click on the Meshed AP in the list of devices and under Overview click “Uplink (Wireless)” or “General”

Adopting the new AP wirelessly
It is possible to adopt the new AP wirelessly as along as step 1 and step 3 are done. It will take longer for the Controller for find the new AP when it is not connected to the network with a wire.

more info
https://lazyadmin.nl/home-network/unifi-wireless-uplink/