I found this old thread (locked now): https://community.tp-link.com/en/business/forum/topic/617912.

I just received a brand new EAP and tried to adopt it to the oc200. I already have one EAP670 and one EAP653 next to omada switches.

I've tried everything and could not get the adoption to work, although the EAP worked in standalone mode and I could upgrade the firmware from the web interface of the EAP. After many reboots and resets I've noticed that in the EAPs web interface/maintenance (or settings)/Controller there is a hyperlink indicating which controller to be informed on the presence. I cannot post a picture anymore, since the EAP is now adopted :(.

The link contained 0.0.0.0 and I've changed that to the IP of the controller: 192.168.0.250 in my case and kept the rest of the syntax as it was (long string).

Then I moved once more to the controller interface and retried the adoption and it worked - this failed in my previous attempts like 10-15 times.