@PeteT 

Hey

Two things come to mind that would stop it from being detected.

Is this a mesh AP, namely its not connected via a LAN connection?   If so you might need to enable MESH on your controller to get this to adopt.  Its unders settings, site, advanced features.    

Is the firmware correct?   If you are running v5 controller is the AP running the correct firmware (should be 5.x firmware).   If you plug this into the router/switch via LAN does it get an IP?

  @PeteT 

Try a shorter Ethernet cable to connect the EAP225-ou.

  @PeteT 

How are you powering your EAP225 outdoor (via the included passive PoE adapter or via a PoE switch), and does its LED indicator come on? If so, can you connect to it in standalone mode (see e.g. https://www.tp-link.com/us/configuration-guides/quick_setup_guide_for_standalone_omada_eaps/)?

If the EAP is actually working: while you need a compatible firmware version for your EAP in order for it to be adopted by your OC200 controller, it should still be shown in the list of available devices. If your EAP225 outdoor is in a different subnet than your controller, try running the Omada discovery utility (see https://www.tp-link.com/de/support/faq/2533/).