In the end I just replaced the AP with a new one and that worked first time with the existing cabling so I'm guessing the unit is somehow faulty. I'll just send the original one back.

Installed about 10m of Cat6 cable for an EAP225-Outdoor but can't get AP to work. It powers on and broadcasts SSID but does get an IP address from the router. Swapped it for a standard EAP225 and...