@Bpp 

RTFM?

isolated would seem to indicate it has not received an IP during its initialization process.  Check your wiring, or if you are trying to mesh, make sure your Omada Controller is connected and performing to spec.  TPlink APs mesh on 5.8G only, and REQUIRE a controller in order to do so.  Once your wired AP is recognized and managed by the Controller you can reset and power on your remote AP.  It may take a number of minutes to discover and show the device in the controller in ADOPT? state.

  @Bpp I assume you have another wifi enabled router or modem/router that you are augmenting with the EAP225?  If so, don't expect seamless roaming between the two, and you won't even be able to connect both if they have slight differences in the WiFi encryption settings (like the WPA/WPA2 and TKIP/AES settings specifically).

If you can't make a longer cable yourself, then I would purchase either a longer cable, or a better coupler (which could just be an old powered ethernet switch from your parts bin).