Hello Alexander,
The AP650 APs have been EOL for a while now, the first thing you need to verify is the firmware version since those have been around since the 4.x firmware days and will not automatically adopt to and upgrade on a 5.x controller.
Start by checking the firmware on both
Check the method used to adopt existing AP650 APs (option 191, controller host, mlcp vlan)
Thank you,
Chris
Christoph S.