Sorry for the late reply - we just had a baby!If your Orbi router is in AP mode I'm not sure how you would have the access to enable access control in stock as that is a routing feature. Is it possible that you didn't have it AP mode in stock?
Yes, it was definitely in ap mode. Most features were greyed out and couldn't be used but access control was still enabled and accessible from a link at the top right of the attached devices screen. It was definitely working too as guests who joined our network were showing as 'denied' in the attached devices screen until I set them to 'allowed' under access control. I thought access control was part of WiFi security, not routing as it was more in line with who can join your wireless rather than what they can do once they have joined?